You could also customize a cable instead of the pedal
I have a Dunlop Volume[X] which i use as an expression pedal.
To get it functional with the GT/22 I opened it up and changed the setting on the little blue switch. It works fine now.
However my Mission SP-1 may need a re-solder on the output jack to change the polarity. It's pretty much useless the way it is wired now.