This script will build output to the screen a properly formatted XML help file. I based this on one of the installed helpfiles in System32.

New-HelpFile.ps1

<#
.SYNOPSIS
    This script will generate a proper XML helpfile
.DESCRIPTION
    This script will build output to the screen a properly formatted
    XML help file. I based this on one of the installed helpfiles
    in System32.
.PARAMETER Commands
    You can pass in the output from Get-Command cmdletname or you 
    can pass in Get-Command -Module moduleName
.EXAMPLE
    New-HelpFile -Commands (Get-Command Get-Help) |out-File .\gethelp.psm1-help.xml
.EXAMPLE
    New-HelpFile -Commands (Get-Command -Module PowerShellAccessControl) |out-File .\PowerShellAccessControl.psm1-help.xml
.NOTES
    ScriptName : New-HelpFile
    Created By : jspatton
    Date Coded : 03/04/2015 17:19:27
.LINK
    https://gist.github.com/jeffpatton1971/029e58a3304cf5761f2e
#>
[CmdletBinding()]
Param
(
$Commands
)
Process
{
    Write-Output '<?xml version = "1.0" encoding = "utf-8" ?>'
    Write-Output ''
    Write-Output '<helpItems schema="maml">'
    Write-Output '  <command:command xmlns:maml="http://schemas.microsoft.com/maml/2004/10"'
    Write-Output '                   xmlns:command="http://schemas.microsoft.com/maml/dev/command/2004/10"'
    Write-Output '                   xmlns:dev="http://schemas.microsoft.com/maml/dev/2004/10"'
    Write-Output '                   xmlns:MSHelp="http://msdn.microsoft.com/mshelp">'
    foreach ($Cmdlet in $Commands)
    {
        $CmdletHelp = Get-Help -Name $Cmdlet.Name -Full;
        Write-Output '      <command:details>'
        Write-Output "          <command:name>$($CmdletHelp.Name)</command:name>"
        Write-Output '          <maml:description>'
        Write-Output "              <maml:para>$([System.Net.WebUtility]::HtmlEncode($CmdletHelp.Synopsis))</maml:para>"
        Write-Output '          </maml:description>'
        Write-Output '          <maml:copyright>'
        Write-Output '              <maml:para />'
        Write-Output '          </maml:copyright>'
        Write-Output "          <command:verb>$($Cmdlet.Verb)</command:verb>"
        Write-Output "          <command:noun>$($Cmdlet.Noun)</command:noun>"
        Write-Output '          <dev:version />'
        Write-Output '      </command:details>'
        Write-Output '      <maml:description>'
        $Desc = $CmdletHelp.Description.Text
        if ($Desc -eq $null)
        {
            $Desc = ""
            }
        if (($Desc).IndexOfAny("`r`n"))
        {
            $Description = ($Desc).Split("`r`n")
            foreach ($DescriptionText in $Description)
            {
                Write-Output "          <maml:para>"
                Write-Output "          $([System.Net.WebUtility]::HtmlEncode($DescriptionText))"
                Write-Output "          </maml:para>"
                }
            }
        else
        {
            $DescriptionText = ($Desc)
            Write-Output "          <maml:para>"
            Write-Output "          $([System.Net.WebUtility]::HtmlEncode($DescriptionText))"
            Write-Output "          </maml:para>"
            }
        Write-Output '      </maml:description>'
        Write-Output '      <command:syntax>'
        $Syntax = $CmdletHelp.syntax
        foreach ($SyntaxItem in $Syntax.SyntaxItem)
        {

            Write-Output '          <command:syntaxItem>'
            Write-Output "              <maml:name>$($CmdletHelp.Name)</maml:name>"
            foreach ($siParam in $SyntaxItem.Parameter)
            {
                Write-Output "              <command:parameter required=`"$($siParam.required)`" variableLength=`"$($siParam.variableLength)`" globbing=`"$($siParam.globbing)`" pipelineInput=`"$($siParam.pipelineInput)`" position=`"$($siParam.position)`" aliases=`"$($siParam.aliases)`">"
                Write-Output "                  <maml:name>$([System.Net.WebUtility]::HtmlEncode($siParam.name))</maml:name>"
                Write-Output '                      <maml:description>'
                $Desc = $siParam.Description.Text
                if ($Desc -eq $null)
                {
                    $Desc = ""
                    }
                if (($Desc).IndexOfAny("`r`n"))
                {
                    $Description = ($Desc).Split("`r`n")
                    foreach ($DescriptionText in $Description)
                    {
                        Write-Output "                          <maml:para>"
                        Write-Output "                          $([System.Net.WebUtility]::HtmlEncode($DescriptionText))"
                        Write-Output "                          </maml:para>"
                        }
                    }
                else
                {
                    $DescriptionText = ($Desc)
                    Write-Output "                          <maml:para>"
                    Write-Output "                          $([System.Net.WebUtility]::HtmlEncode($DescriptionText))"
                    Write-Output "                          </maml:para>"
                    }
                Write-Output '                      </maml:description>'
                Write-Output "                  <command:parameterValue required=`"true`" variableLength=`"true`">$($siParam.parameterValue)</command:parameterValue>"
                Write-Output '              </command:parameter>'
                }
            Write-Output '          </command:syntaxItem>'
            }
        Write-Output '      </command:syntax>'
        Write-Output '      <command:parameters>'
        foreach ($cParam in $CmdletHelp.parameters.parameter)
        {
            Write-Output "          <command:parameter required=`"$($cParam.required)`" variableLength=`"$($cParam.variableLength)`" globbing=`"$($cParam.globbing)`" pipelineInput=`"$($cParam.pipelineInput)`" position=`"$($cParam.position)`" aliases=`"$($cParam.aliases)`">"
            Write-Output "              <maml:name>$($cParam.name)</maml:name>"
            Write-Output '                  <maml:description>'
            $Desc = $cParam.description.text
            if ($Desc -eq $null)
            {
                $Desc = ""
                }
            if (($Desc).IndexOfAny("`r`n"))
            {
                $Description = ($Desc).Split("`r`n")
                foreach ($DescriptionText in $Description)
                {
                    Write-Output "                      <maml:para>"
                    Write-Output "                      $([System.Net.WebUtility]::HtmlEncode($DescriptionText))"
                    Write-Output "                      </maml:para>"
                    }
                }
            else
            {
                $DescriptionText = ($cParam.description.text)
                Write-Output "                      <maml:para>"
                Write-Output "                      $([System.Net.WebUtility]::HtmlEncode($DescriptionText))"
                Write-Output "                      </maml:para>"
                }
            Write-Output '                  </maml:description>'
            Write-Output "                  <command:parameterValue required=`"$($cParam.parameterValue.required)`" variableLength=`"$($cParam.parameterValue.variableLength)`">$($cParam.parameterValue)</command:parameterValue>"
            Write-Output '                  <dev:type>'
            Write-Output "                      <maml:name>$($cParam.type.name)</maml:name>"
            Write-Output "                      <maml:uri>$($cParam.type.uri)</maml:uri>"
            Write-Output '                  </dev:type>'
            Write-Output "                  <dev:defaultValue>$($cParam.defaultValue)</dev:defaultValue>"
            Write-Output '          </command:parameter>'
            }
        Write-Output '      </command:parameters>'
        Write-Output '    <command:inputTypes></command:inputTypes>'
        Write-Output '    <command:returnValues></command:returnValues>'
        Write-Output '    <maml:alertSet>'
        foreach ($AlertSet in $CmdletHelp.alertSet)
        {
            Write-Output "      <maml:title>$($AlertSet.Title)</maml:title>"
            foreach ($aSet in $AlertSet.alert)
            {
                Write-Output '      <maml:alert>'
                Write-Output "        <maml:para>"
                Write-Output "$([System.Net.WebUtility]::HtmlEncode($aSet.Text))"
                Write-Output "        </maml:para>"
                Write-Output '      </maml:alert>'
                }
            }
        Write-Output '    </maml:alertSet>'
        Write-Output '    <command:terminatingErrors /><command:nonTerminatingErrors />'
        Write-Output '    <command:examples>'
        foreach ($cExample in $CmdletHelp.examples.example)
        {
            Write-Output '        <command:example>'
            Write-Output "            <maml:title>$($cExample.title)</maml:title>"
            Write-Output '            <maml:introduction>'
            Write-Output "                <maml:para>$([System.Net.WebUtility]::HtmlEncode($cExample.introduction.Text))</maml:para>"
            Write-Output '            </maml:introduction>'
            Write-Output '            <dev:code>'
            Write-Output "$($cExample.code)"
            Write-Output '            </dev:code>'
            Write-Output '            <dev:remarks>'
            Write-Output "                <maml:para>$([System.Net.WebUtility]::HtmlEncode($cExample.remarks.Text))</maml:para>"
            Write-Output '            </dev:remarks>'
            Write-Output '            <command:commandLines>'
            foreach ($cmdLine in $cExample.commandLines)
            {
                Write-Output '                <command:commandLine>'
                Write-Output "                    <command:commandText>$($cmdLine.commandLine.commandText)</command:commandText>"
                Write-Output '                </command:commandLine>'
                }
            Write-Output '            </command:commandLines>'
            Write-Output '        </command:example>'
            }
        Write-Output '    </command:examples>'
        Write-Output '    <maml:relatedLinks>'
        foreach ($navigationLink in $CmdletHelp.relatedLinks.navigationLink)
        {
            Write-Output '        <maml:navigationLink>'
            Write-Output "            <maml:linkText>$($navigationLink.linkText)</maml:linkText>"
            Write-Output "            <maml:uri>$($navigationLink.uri)</maml:uri>"
            Write-Output '        </maml:navigationLink>'
            }
        Write-Output '    </maml:relatedLinks>'
        }
    Write-Output "  </command:command>"
    Write-Output "</helpItems>"
    }

I have skipped some items in the helpfile, I'm basically using this to build xml helpfiles for my powershell script modules. I wrote fairly complete inline help in those, so I can pass in the module and it will output the helpfile. You will need to do some editing as it's not perfect, if some of the output from examples and so on is formatted as XML you will need to htmlencode that using some utilty. I used http://www.webtoolhub.com/tn561387-html-encoder-decoder.aspx as it worked well enough for my needs.

I do the htmlencode now...just in case

Fixed some annoying issues, I have a tendnancy to insert carriage returns in my inline help, and it appears when it gets parsed by the get-help cmdlet it doesn't break it up into multiple maml:para/maml:para blocks, so I remove rn where I find it in the three description fields.

Also fixed it so when you pipe to out-file it actually pipes everything, the last two entries were write-host which doesn't pipe to out-file.

split those carriage returns into separate maml entries

To make sure that your help works, make sure you decorate your module file based on the answer in this thread.
http://stackoverflow.com/questions/1432717/powershell-v2-external-maml-help

This is the decoration
<#
.ExternalHelp C:<><>\en-US<>.help.xml
#>