With this script you can add a custom prompt for playing a message while a call is being transfered to another queue. This script will import a prompt (25.wav) from the directory C:\temp and play the prompt when calls will "overflow" from one queue to another.

$x = Get-CsRgsQueue -Identity service:ApplicationServer:lyncpool01.contoso.com -Name “Sales_Overflow”
$z = Get-CsRgsQueue -Identity service:ApplicationServer:lyncpool01.contoso.com-Name “Sales”

$musicFile = Get-Content -ReadCount 0 -Encoding Byte “C:\Temp\25.wav” | Import-CsRgsAudioFile -Identity Service:ApplicationServer:lyncpool01.contoso.com -FileName “25.wav”
$prompt = New-CsRgsPrompt -AudioFilePrompt $musicFile

$y = New-CsRgsCallAction -Prompt $prompt -Action TransferToQueue -QueueID $x.Identity

$z.OverflowAction = $y
Set-CsRgsQueue $z