Create project specific datasource templates for your SXA tenant so you can extend them with custom fields - Promo on Showcase example

Add-ProjectSpecificComponentDatasource.ps1

###### user settings start here

# Path to the tenant that you want to update
$tenantTemplatesPath = "master:\templates\Project\Showcase"

# Site that should be upgraded to the new template - ise asterisk to upgrade multiple sites.
$sitePath = "master:\content\Showcase\*"

# Path to the component for which you want to introduce your own data source
$componentPath = 'master:\layout\Renderings\Feature\Experience Accelerator\Page Content\Promo'

# Path to the feature or foundation folder that contains the templates
$featurePath = "Feature/Experience Accelerator/Page Content"

# templates to be cloned
$datasourceTemplateName = "Promo"
$datasourceFolderTemplateName = "Promo Folder"

###### user settings end here

# The function creates a Template in the project based on a template in either Foudation or Feature layer
function Assert-FeatureTemplateInProject {
     [CmdletBinding()]
    param(
        [parameter(Mandatory=$true)]
        [String[]] $TenantTemplateFolder,
        [parameter(Mandatory=$true)]
        [String[]] $FeatureTemplatePath,
        [parameter(Mandatory=$true)]
        [String[]] $FeatureTemplateName
        )
        
    # does the template exist?
    $projectTemplate = Get-Item "$tenantTemplateFolder\$featureTemplateName" -ErrorAction SilentlyContinue

    #if we haven't created tenant events yet - we need to do it
    if ($projectTemplate -eq $null){
        # create template
        $projectTemplate = New-Item "$tenantTemplateFolder\$featureTemplateName" -ItemType "System/Templates/Template"
        # add base feature template
        Add-BaseTemplate  -Item $projectTemplate -Template "$featureTemplatePath/$featureTemplateName"
        # copy template icon from the base template
        $projectTemplate.__Icon = (Get-Item "master:\templates\$featureTemplatePath/$featureTemplateName").__Icon
    }

    #create standard values if they don't exist
    [Sitecore.Data.Items.TemplateItem]$templateItem = $projectTemplate
    if($TemplateItem.StandardValues -eq $null){
        $standardValues = $TemplateItem.CreateStandardValues()
    }

    # return the template
    return $projectTemplate
}

# The function adds a template to the Insert options of another template on Standard values
function Add-ToStandardValues {
     [CmdletBinding()]
    param(
        [parameter(Mandatory=$true)]
        [Item] $Template,
        [parameter(Mandatory=$true)]
        [Item[]] $InsertTemplates
        )
        
    # let's get the standard values
    [Sitecore.Data.Items.TemplateItem]$templateItem = Get-Item "$($Template.ProviderPath)"
    #if we haven't created project calendar event yet - we need to do it
    if($TemplateItem.StandardValues -eq $null){
        $standardValues = $TemplateItem.CreateStandardValues()
    }else{
        $standardValues = Get-Item "$($Template.ProviderPath)\__Standard Values" -ErrorAction SilentlyContinue
    }
    $standardValues.__Masters = $InsertTemplates
}

# copy templates to project
$datasourceTemplate = Assert-FeatureTemplateInProject -TenantTemplateFolder $tenantTemplatesPath -FeatureTemplatePath $featurePath -FeatureTemplateName $datasourceTemplateName
$datasourceFolderTemplate = Assert-FeatureTemplateInProject -TenantTemplateFolder $tenantTemplatesPath -FeatureTemplatePath $featurePath -FeatureTemplateName $datasourceFolderTemplateName

# set insert options
Add-ToStandardValues -Template $datasourceFolderTemplate -InsertTemplates $datasourceTemplate

# replace the data source template with project specific
Get-ChildItem "$sitePath\Data" -Recurse | ? { $_.TemplateName -eq $datasourceTemplateName } | 
  % { Set-ItemTemplate -Item $_ -TemplateItem $datasourceTemplate;} 

# replace the data source folder template with project specific
Get-ChildItem "$sitePath\Data" -Recurse | ? { $_.TemplateName -eq $datasourceFolderTemplateName } | 
  % { Set-ItemTemplate -Item $_ -TemplateItem $datasourceFolderTemplate; $_.__Masters = @($datasourceFolderTemplate, $datasourceTemplate); } 

# get component
$component = Get-Item $componentPath

# Create datasource configuration for the component
$datacourceConfigRoots = @() + (Get-Item "$sitePath\Settings" | %{ Get-ChildItem $_.ProviderPath -Recurse }  | 
  ? { Test-BaseTemplate -Item $_ -TemplateItem (Get-Item master: -ID "$([Sitecore.XA.Foundation.LocalDatasources.Templates+DatasourceConfigurations]::ID)") } )

foreach($datacourceConfigRoot in $datacourceConfigRoots)
{
    if(Test-Path "$datacourceConfigRoot\$($component.Name)"){
        Write-Host "Configuration Exists - aborting"
        continue
    }
    $datasourceConfiguration = New-Item -Path $datacourceConfigRoot.ProviderPath -Name "$($component.Name)" -ItemType ((Get-Item master: -ID "$([Sitecore.XA.Foundation.LocalDatasources.Templates+DatasourceConfiguration]::ID)").Paths.Path)
    $datasourceConfiguration.DatasourceTemplate = $datasourceTemplate.Paths.Path
    $datasourceConfiguration.CompatibleRenderings = $component
}

Thanks Adam for this, very helpful. Minor change/suggestion to this script:

# set insert options
Add-ToStandardValues -Template $datasourceFolderTemplate -InsertTemplates $datasourceFolderTemplate
Add-ToStandardValues -Template $datasourceFolderTemplate -InsertTemplates $datasourceTemplate

In the example above, this ensures that child Promo Folders and Promos are set via Insert Options.