..in my case way more error than trial
As for what you say about the expression pedal I have noticed a parameter in the SYSTEM menu "MIDI > RCV CTL" which allows you to set PEDAL to a CC" – have you tried that yet?
It suggests to me that you could set up that PEDAL parameter to different CC numbers on each pedal and then try out the MIDI controller expression pedal to those CCs and see if you can access one or more stompbox's knob parameters *as if* there were an expression pedal attached.
Well, that's optimism for you. Good luck let me know how it goes