You cannot use a Line 6 FBV connected to the back of the Kemper.
MIDI pedals can control it. Not a stock FBV pedal.
Don't own one but I glanced at the manual for it and it looks like you can program it to send out any midi messages you want so yes it will work with the Kemper once re-programmed. Having only 4 switches will limit you quite a bit though, I just bought a FCB1010 for $100 to go with my Roland FC-300 and I might finally have enough switches at 25 lol. Enjoy your Kemper!
Don't own one but I glanced at the manual for it and it looks like you can program it to send out any midi messages you want so yes it will work with the Kemper once re-programmed. Having only 4 switches will limit you quite a bit though, I just bought a FCB1010 for $100 to go with my Roland FC-300 and I might finally have enough switches at 25 lol. Enjoy your Kemper!
An FBV can't be programmed to send MIDI messages on its own. If it is connected to Line 6 gear, the FBV can be used to program the Line 6 gear to send the MIDI messages, if the Line 6 gear has a five pin MIDI Out connector.
I should have done a longer glance 
There was some sort of mod kit to make the FBV compatible with other devices, but you're better off buying something like an FCB. Or even holding out for the Kemper Foot Controller, which seems to be just over the horizon.
