Hey there. Yet another nooby question from yours truly.

Can the Boss FV500H volume pedal be used as an expression pedal?
It most certainly can. It even has a specific output for that. And I might add, BUY ONE. They are amazing pedals, the only downside is they take up a big chunk of your pedalboard. I just keep mine on the side, off of my pedalboard. They're insanely cheap for as long lasting and durable they are.
Telecasters-->Dunlop Volume Pedal-T1M Pearl-AMT Japanese Girl Wah-Line 6 M9-Ibanez DE7-EHX Cathedral-->Mesa Boogie Nomad 4x10 combo
Just sure you buy it somewhere that allows returns. I checked the manual for the FV500H and it's vague about the impedance. In my experience some devices like expression pedals to have pots in the range of 10-25K and don't work well with high impedance pots. The FV500H is a high impedance pedal but the manual offers no specs.