user defined workbench exit code

Not sure if I understand correctly. Do you mean when run as batch from commandline?

Usually a workbench (if succesfully) exit's with an exit code 0. This code can be "read" in a batch-script (it is %ERRORLEVEL%). I would like to (succesfully) exit it with my own (user defined/specific) exit code, not being 0.

It would be an errorlevel generated by fme.exe and I don't think those can be altered by the user. Perhaps you can do something with the Terminator and a custom termination message?