A: Not all expression pedals are compatible. In some cases, you may not get the full range of values for a parameter controlled through the pedal, and in others, the wiring may not be compatible. Having said that, there are several options. For a true expression pedal, we recommend the Roland EV-5.
You can also use a volume pedal that contains a 25k potentiometer as an expression pedal for the M13. Two volume pedals that are reported to work with the M13 as expression pedals are the BOSS FV-500H (turn the “minumum” knob all the way up for best results) and FV-500L.