The Remote 4 is designed to be both simple and DIY-friendly, so it does not support an expression pedal at the same time.  For cases where you need the footswitches and an expression pedal, a MIDI controller with an expression pedal input is the best approach.  Connect the MIDI controller to your pedal using either USB or TRS MIDI.

More info:

If you let us know which controller(s) look like they have the layout and features you would want, we can offer some specific suggestions and setup instructions.