It is possible to do like that you said.
For example :: to get "mp3 module" / "voice command module" etc ==> merge them all to original f/w ==> " deluxe firmware " (you may heard about that from somewhere. )
But it based on the original boot.bin
To change f/w to another NUvi model is a very exiting way but who want to get a dead Nuvi after that .
(Actually I know it's possible to change f/w with other nuvi model but... it's only a special way for specific model., normally you will brick nuvi)