Well, I was doing the keybed as part of a much larger refit including MIDI. So, I bought a Fatar Keybed off Keyparts UK (http://www.keyparts.co.uk/) as well as their keybed scanning electronics (which output MIDI). Internal to my synth, I jacked the MIDI from the keybed scanner into my Polysix's MIDI controller.
If you haven't MIDI'fied your Polysix, it'll be harder to do....you'd need a keybed with the same kind of keyscanning connections as the stock keybed. You'd have to dive into a bunch of schematics to find that out.