It's actually not all that hard to convert an existing pedalboard into a touch-sensitive MIDI controller. You can get a 37-note StudioLogic MIDI keyboard for around $100, and by opening it up and soldering to the underside of the circuit boards, it's possible to connect each key contact to the contacts of the pedalboard. The secret lies in the four distinct ground wires: you must cut the ground wire of the pedalboard into four sections, each supplying eight pedals and each connected to the appropriate wire on the keyboard (it's easy to figure out which is which by examining the paths of the circuit board traces).

To produce the touch-sensitive effect (useful if you'll be controlling synthesizers instead of / in addition to a pipe organ), you'll need a pedalboard on which each pedal's contact connects with multiple silver pins when the pedal is depressed. Simply bend down one pin for each note so that it will be contacted a little later than the others, and then run two signal wires for each pedal, one from a standard-position pin and one from the bent pin. The difference between the times the two pins touch the pedal contact determines the note velocity! You will need to play with the exact amount that each pin is bent in order to get the best touch-sensitive response. It would even be possible to have several different sets of pins bent at varying angles, with a master switch to select between them - if you have the patience to do that much wiring! Be sure to either install a bottom cover to protect the contact rail and pins, or raise the pedalboard up off the floor by attaching small blocks to its four corners; otherwise, the pins that you spent so much time calibrating could be bent out of alignment, especially if the pedalboard is sitting on top of carpeting.

 

The end of the 37-key MIDI keyboard, where the signal cables from the pedalboard attach. Note the sturdy wooden frame to which the 25-pin jacks are screwed - it's important to make sure that the jacks can't move around, or the wires soldered to the backs can break or short.

 

The interface box I attached to the pedalboard. Each of the four distinct common wires is connected to the outside shield of one of the plugs installed in the black box. It's important to label both the jacks and the cables to prevent incorrect connections that result in bizarre MIDI errors.

 

The contact rail, modified for touch-sensitivity. I've bent all the unnecessary contacts out of the way and adjusted the two remaining contacts to be hit by the pedal's common contact at slightly different times; this provides the keyboard with enough information to generate MIDI velocity information. The two white wires, the yellow wire, and the purple wire are the four common wires.