If you have a supply of 5V feeding the GarageMote, that should make things easy.
Once you go to 5V you need some level shifting for best practices.
I was thinking of making some kind of microphone/piezo based Mote that can listen/learn the beeps of such an alarm (smoke, CO etc). It then has its own battery, is low power, and you can attach it to any alarm in the home or garage (one should be sufficient for smoke for instance, if they are wired together in the house). How about that?