Sorry, not that simple.

Among other things, these little buggers don't send the same signal every time - as a security precaution against exactly what you're talking about.

Anything you might want to hook up to an antenna has to be a transmitter, and it has to not only transmit on the same freq., but also using the same protocol.

Not to mention any legal infringements you might be causing because the mfr. intends their security system to be, well, secure.