You can buy, direct from Roland, as a spares item, the pedal supplied for their PK-9 Bass Pedals, which is a volume/expression pedal with two auxiliary toe switches for about 50GBP. You will need to cut off the DIN Plug and meter out the separate wires and polarity that control the volume/expression impedance and the two (left and right ) ON/OF toe switches and fit jack plugs to them. (Note impedance is 10-12K ohm whereas the FC7 volume pedal is 50k)
Using the toe switches for sustain and registration changes maybe?
I've observed some Pro players use this pedal modified in this way on their Tyros/Genos organ set ups.
I do too.