How do I support an attachable keyboard on ipad when building a touch keypad?

I'm currently developing a touch keypad for iPhone/iPad ( like the one you use to unlock your device ). But I'm having trouble finding documentation on how I can support a 3rd party attachable keyboard for iPad, and make sure it's inputs translate to the touch pad.

Does anyone know what delegates / classes / documentation I can look at to do this?

Thanks, cpb427

(what I'm talking about )