*disclaimer* I don't have one (yet) but this is what I've gathered from the manual and just generally gas'ing over the elements
The expression input overrides the mix knob when plugged in, and gives you full control over the "mix".
As for expression pedals, you'll want something in the 50k range like the:
Moog Ep2, Boss FV500L (low impedance version), Korg EXP-2
There are some that call me...morningstaru?