liys: the one i use is called a "Lettering Sheet", but I think its most probably like "Rub-On" Stencils or something....
If you want an external expression pedal to replace a pot acting as a resistor, you'll need 2 paths of signal (because the pot actign as the resistor has 2 nodes), so a stereo jack can most probably do the job. There are jacks (mono and stereo) that has switching as well, so like you can make it auto plug-in and it'll switch to the expression pedal. If the pot is acting as a potentiometer in the pedal, you'll need 3 signal paths and you can do this again with a stereo jack but you'll have to isolate the sleeve from the enclosure, because the enclosure is usually grounded.
You'l lhave to make sure the expression pedal you are using is appropriate too! (value&taper&direction of sweep)
Han