You need a mandatory footswitch, I build my own for $10 but anyone will work, liking the Boss FS-5U on TS cable or FS-6 on TRS cable. However you cannot use an expression pedal and a footswitch at the same time, it's one or the other.

What I do, is plug my expression pedal in my midi footpedal and use a 2-footswitch pedal in Eleven Rack for instant access (distortion and modulation), why limit to only 1 when you can connect 2 ;-)