The pinout of the expression pedal should be TIP=output, RING=input, SLEEVE=ground. There's a list of expression pedals on this page that work with the control port. They should also work with the blend exp input. Volume pedals probably have too high of a pot resistance to work with the blend exp port, but you can give it a shot!
Mission EP-1 and EP-2 (no polarity switch, polarity switch, respectively)
M Audio EX-P (has polarity switch)
Moog EP-2 (we've tested the EP-2, and it works. it has been replaced by the EP-3 which we haven't tested yet)