I was trying to do what the OP is describing--using the Wham Pitch effect to drop the pitch down a whole step w/ the exp pedal, and ALMOST got it.
I'm doing this in stompbox mode. So I set up the Wham Pitch on a block, and assigned that block to a footswitch so that I can turn it on and off at will. Then, with the effect on (not bypassed), hit the menu button, then hit button 2 (controller assign), then hit button 6 (learn controller), and move the exp pedal so that it "learns" it. Then, on that same screen, set "min value" to 0 and "max value" to -2.
So this basically does what the OP describes, BUT...the pitch shifting is not smooth. I can hear it drop a half step, and then another half step, but it's not smooth. Anyone know if there's a way to make it a smooth dive?
EDIT 12/5: I figured it out in HX Edit. Not sure why it was so hard to figure out, but I'm new to the Helix. Here's how I did it in stomp box mode...YMMV.
1. Enable the Pitch/Wham block, assign the bypass to the footswitch of your choice.
2. In the Edit window, set the heel & toe values to 0 and -4 (-4 gives you 2 whole steps)
3. Go to the Bypass/Controller assign window, look for Pitch Wham in the list, set the "Pedal Position" parameter to EXP 1 (or 2, whichever you want).
4. The "Bypass" parameter should also be there in the list if you assigned it to a switch (step 1). If you see any other paramaters with assignments, clear them.