I just whipped up an expression pedal for my HD500, with a wah, one additional pot, and even added custom grip to the treadle.  Used a Dunlop GCB-95, a 75K pot, and adhesive diamond pattern grip tape for the top.  I cut out the guts(PCB, and left the stock pot intact.  The 75K pot was wired in parallel with the wah pot, and I installed a standard mono jack in one of the holes left by the stock jacks(they were board mounted, so I couldn't use them).  The additional pot was adjusted so that the Pod would see 10K at heel down, and 0 at toe down.  Wired it all the jack, and it works great!  Just taped the secondary pot to the inside of the case, and if I decide to use the pedal with another processor that requires a different range, I can just adjust that pot to the required resistance.  Hope this helps someone who doesn't have the cash for a custom made expression pedal.  I think I saw the 2 potentiometer trick on another guitar forum, but I can't remember where.