I have a MODX8 and would like to change scenes with my pedal.

Yes, you can change Scenes with any CC number, 1-95. The command must be sent IN via MIDI, however. A programmable MIDI Foot Switch/Pedal could be used. For example, an MFC10 type device would be ideal for such a thing.
Link: Yamaha MIDI Foot Controller MFC10.

It has enough pedal switches to do the job, nicely.
[UTILITY] > “Settings” > “MIDI I/O” > “Scene CC”
Set the value here to the same value as your Foot pedal.
All values are as follows
Scene 1: 0-15
Scene 2: 16-31
Scene 3: 32-47
Scene 4: 48-63
Scene 5: 64-79
Scene 6: 80-95
Scene 7: 96-111
Scene 8: 112-127

Translation: Any value between 0 and 15 will select Scene 1, and so on.

Seems so far it only changes live sets and changing scenes on the fly would make for simple programing.

I don’t agree but I guess we each have our own preferences. I find the buttons just fine. If you were thinking about using a Foot Switch plugged directly to one of the MODX’s Assignable Foot pedal or Foot Switch jacks, that is not available. Although the button presses can send a CC message Out via MIDI, the MODX does not address itself with MIDI. What you can do: you must capture the data with a device like a Sequencer which could play it back to to MODX via MIDI. You can automate Scene changes using the Performance Recorder.

Thank you for the info. The foot controller would add another dimension to playing. Have a folder 1 1/2 inches thick from your solutions and insights from learning my motif. Thanks Again.