How do I set environment variables for a portable app that already exists?

I can't use launcher.ini because that is only for making a NEW portable app, not configuring an existing one. I've already tried creating a BAT file contains the environment variable setting command in one line like "set PATH=%PATH%;C:\MyPath", and then executes the portable app launcher in next line like "FirefoxPortable.exe". Unfortunately it seems that environment variables set in a BAT file, do not get passed from the portable app launcher to the actual app's EXE file (in this case firefox.exe).

Is there ANY way to set an enviornment variable for an already existing portable app? Or is this something that would now involve outright hacking the portable app launcher EXE file?