Chain a PowerShell script block to the tabexpansion2 function, calling the original implementation if the new one doesn't return a result.

Add-TabExpansion.ps1

function Add-TabExpansion
{
    param([ScriptBlock][parameter(mandatory)]$ScriptBlock)

    $originalFunction = (gcm TabExpansion2).ScriptBlock
    $chainedBlock = {
        $userResult = & $ScriptBlock @Args
        if($userResult) {
            return $userResult
        }
        return & $originalFunction @Args
    }.GetNewClosure()
    Set-Content Function:\TabExpansion2 $chainedBlock
}

Add-TabExpansion @Args