From the EV manual....
"Do NOT use Expression and Volume functions at the same time or ground loops can occur."
Even If you could use the 'physical' volume pedal function of the EV-2 and the expression pedal function at the same time, the volume pedal would always be active, preventing you from using it as an expression pedal without the volume also being affected.
You best bet is to use it as an expression pedal. You can set it up to be whatever you want it to be, once you get your head wrapped around how it works. Or get a second pedal.. One dedicated to volume, and the other for everything else.