For an existing file to tape job its possible to set the file selection with a powershell script. It's been a while since I've used it, so I cannot guarantee that it still works. And I'm not at my computer so formatting may not look that good.
$files = ListofFiles
#Generate "FiletoTape"-Objects
$filestotape = foreach($f in $files){New-VBRFileToTapeObject -Path $f.fullname -Server (Get-VBRServer -name VBRServer) -ErrorVariable +vbrerror}
#Replace Objects in File to tape job
Get-VBRTapeJob -name JobName -ErrorVariable +vbrerror | Set-VBRFileToTapeJob -Object $filestotape -ErrorVariable +vbrerror