It also kind of depends what you mean by a theremin. It can mean specifically an instrument that produces sound as a beat frequency from heterodyning circuitry (I hope I got that right!) and generally one played by gesturing near to one or preferably two sensing devices, preferably antenna.
I'm not an electronics whizz, so I can't help you with converting it to either, but I do know that heterodyning is related to radio transmission, so I guess the question is - are you considering a deep hack to modify existing heterodyning circuitry within your receiver and make a theremin in the specific sense, or are you looking for a novel way of twiddling the knobs, so to speak, by strapping a theremin-like user interface onto it?
Gordon