Theremin MIDI interface help?

So...I'm new to the world of Arduino and I'm absolutely loving all of the possibilities. My first build is the Open.Theremin.UNO (

I'm just waiting on the rest of my components to complete the build. But my bandmate had the idea that we hook up the theremin to his synth and use it for pitchbends so we're trying to figure out what kind of interface we would need in order to do that. In order to pitchbend a synth we'd need MIDI...right? And if that's the would we go from the theremin's stereo output to a MIDI?

I can follow build plans and schematics relatively well and can alter code (but not write it myself [yet]) but figuring out how to build...whatever I would need for this all by lonesome is where I get hung up. I don't even know where to start. Any help would be greatly appreciated!

(Also, I downloaded these plans and that seems to be close to what we want to achieve but we don't need all the fancy stuff on top of that but I'm having trouble altering the if anyone has good technical know-how on modifying stuff like that, that would be super helpful...Another option would be just figuring out how to hook up these components to an arduino because the board he has it on is basically a homemade arduino.)