Bad Stone Expression Mod

Okay so I really like my bad stone, especially the manual shift mode. I would like to mod the circuit to where the manual shift knob could be controlled with an expression pedal, in the same way my worm pedal, or a pigtronix envelope phaser, or the mutron biphase, operates. The way I understand it, the manual shift knob is just like any other resistance pot. Is there a way I can add in a jack and use an expression pedal in place of the knob? Any advice would be appreciated