YOU CARRY OUT THIS MOD AT YOUR OWN RISK!!!
It's a little more awkward modifying the micro q-tron than the mini, but still perfectly achievable. Have a look at the pic attached.
First thing you have to do is cut the red track on the footswitch pcb (shown in the diagram) to break the connection between the ribbon cable and the footswitch. If it were me I'd cut it more than once to be sure!
Next get a potentiometer. I'd go for a 1 megaohm audio taper although I've used other values (250k, 500k). The little diagram within the pic shows looking at the pot from the back. The red wire should be connected to the ribbon cable end of the red track. The green wire should connect to the top right of the 9 footswitch pins (the other end of the red track). The 3rd pot connection should be grounded. I'd connect it to the chassis of the pot (if it's a metal case) then to an available ground on the circuit - I'd connect to the sleeve lug on the output jack).
Where you drill the hole for the pot is entirely up to you. I'd be tempted to mount it on the side of the pedal on the footswitch side of the output jack, about where the battery usually sits. There's the same room on the other side of the footswitch to put the battery.