Opening the device would give you something like this:
To re-use the keyboard, you would have to grind off the black blob of epoxy, to remove the chip (old procesor) underneath. When you have done this, the old processor won't interfere with the buttons. Now you can start tracing the connections to the buttons. Good luck
I have done something similar ages ago - i converted a Ericsson SMS-keyboard (the one you plug into the bottom of an old cellphone to an infrared remote. it took many hours, and wasn't really worth the effort
I would say it's a good exercise, if you can't afford a "real" keyboard for a few $'s.