Yes it is possible to control the wah using the MIDI expression pedal,

In the ground control foot controller , you would connect  your expression pedal and you need to assign a channel  over which the Ground Controller would send MIDI CCs out to the devices being controlled.

In the modfactor MIDI settings-> rcv CTL  , select PDL as the target (Left Side) and then select the number that you have chosen on your foot controller as the source(Right Side). Then once you do this , you would need to set your wah to expression pedal and you should be all set.

However you need to be sure that this MIDI channel is not being used by other devices in your MIDI chain otherwise changing values over expression pedal for Wah, might have undesirable effects in those devices.

Hope This Helps.

Amey