I was having a similar issue recently, as I have two Digitech units (a Legend II and an RP-2000), neither of which transmits MIDI expression data. I just traded my RP-2000 for a Boss GT-3, as it seems that the Boss units transmit expression data. I haven't hooked it up to try it yet, but I am reasonably certain that it will work. Your other option would be to get a dedicated MIDI foot controller, like the Behringer FB that you mentioned, or something similar. You should also be able to use your existing expression pedals via your keyboard, as the other poster said, but how to then get that data into Amplitube is a bit of a mystery to me.
I will see if I can't find time to hook up the Boss unit and test it out, and report back.