I dont know about your exact pedal but I tried to solve the exact same situation with my set up, see my other post about it, on the forum, and I couldn't in short!!!
I found that,to use the expression pedal on my floorboard, I had to assign a midi control number to it, which was fine, but then realised that I cant change those midi numbers from patch to patch (oh that I could lol)so it would control EITHER wha or vol.
so I set it to control Wha then acquired another expression pedal (Korg exp2 mainly because it has an orange label lol) which I plug into the expression pedal jack on the back, therefore I CAN assign that to mutiple purposes either Vol, or multi-fx etc from patch to patch, its the most elegant way I can see to achive the results your after without buying a wjole other midi controller with 2 exp pedals (fcb1010 for example)I reccoment the Korg because its robust and you can use a stereo jack of your own to connect it, meaning you don't get lumped with the 2 metre one usually with the units, (there may well be other better units out there, but this was best of 3 dusty ones I found in the store room of a local music shop)