I think the bit your missing might be here:

Function RunSuAs($CommandLine)

$Devpath="\ntreskit\Su.exe"
If Exist($Devpath)

SetL "SU_DOMAIN=FRITZEU"
SetL "SU_PASSWORD=$SuPassword"
SetL "SU_USERNAME=$SuUsername"
SetL "SU_COMMANDLINE=$CommandLine"

SHELL $DevPath ; <------- here

EndIf

EndFunction