The FI accepts only 5-pin MIDI input. An expression pedal has a TRS connector. You need a piece of equipment to choose which MIDI CC message the expression will control and which has a 5-pin MIDI output. A cheap solution is the Source Audio Hub (and a good choice if you have a Source Audio pedal in your setup). Otherwise there are plenty of other choices from Boss, Disaster Area, Line 6, Morningstar etc that will serve your needs. If you don’t need it to be foot controlled, you could simply use an iOS or Android device with a MIDI app and control changes via the touch screen.