this mod may be untested on the x0xb0x but I think it's pretty standard & well-tested when it comes to TB303s. The illustration of this mod which I found on hyperreal is about 10 years old now.
I'll be trying it as soon as I find the right switch.
Oh and I think you want to mod the ext. in to run directly to the third position of the switch, rather than using the existing external in circuitry.
I might even add a diode buffer network to help shield the circuit from potential nastiness... rather than a completely straight wiring from input jack to switch. This might not be necessary, but shouldn't hurt either.