Can either of the foot switch pedals be assigned the volume up/down function.
As far as I know, not in the current firmware.
A solution that would leave the footswitch open for a multitude of other functions would be to insert a volume pedal between the output of the BB and your full range sound system. There very small ones available like the Ernie Ball or AMT or an old Digitech wah/volume. Of course this won’t work if you’re running something else thought the BB. I plan to run mine separately to the pa/amp so drums are on a different channel and can be eq’d separately.