Create a virtual directory that is mapped to a UNC

New-UncVirtualDirectory.ps1

$virtualPath = "vdir"
$physicalPath = "\\server\share"
$parentSite = "Default Website"
$username = "share_user"
$password = "share_password"

Add-PSSnapin WebAdministration -ErrorAction SilentlyContinue
Import-Module WebAdministration -ErrorAction SilentlyContinue

Write-Host "Getting web site $parentSite."
$site = Get-Website -name $parentSite
if (!$site) {
    throw "The web site '$parentSite' does not exist. Please create the site first."
}

Write-Host "Determining whether virtual directory already exists."
$existing = Get-WebVirtualDirectory -site $parentSite -Name $virtualPath

if (-not $existing) {
    Write-Host "Creating virtual directory '$virtualPath'"
    New-Item IIS:\Sites\$parentSite\$virtualPath -type VirtualDirectory -physicalPath $physicalPath
    Write-Host "Virtual directory created successfully."
}
else {
    Write-Host "Virtual directory '$virtualPath' already exists. Moving on..."
}

if ($username -ne $null) {
    Write-Host "Setting credentials on virtual directory for $username"
    Set-WebConfigurationProperty "system.applicationHost/sites/site[@name='$parentSite']/application[@path='/']/virtualDirectory[@path='$virtualPath']" -name username -Value $username
    Set-WebConfigurationProperty "system.applicationHost/sites/site[@name='$parentSite']/application[@path='/']/virtualDirectory[@path='$virtualPath']" -Name password -Value $password
    Write-Host "Virtual directory credentials set successfully."
}