You will want to use the Line 6 EX-1. It plugs into the FBV pedal and will only control the volume (the FBV pedal will then only be a Wah control).
I'm not sure why that page does not indicate it is also compatible with the FBV Express MkII, but it is.
Another expression pedal that some users prefer is the Mission Engineering one. Here's a link to their website/product page: http://www.mission-engineering.com/Mission-Engineering/Line_6.html