Well, I am not sure how the phone works, but it does sound possible to send the empeg outputs into the phone inputs on the factory HU. It depends what kind of interface the phone has to the HU. There may be an adapter out there, but it sounds like this is a somewhat rare setup, so dont hold your breath.
I was in a similar dilemma when I installed my empeg. Wanted to keep factory HU for looks, internal amp, radio, and CD. It could not accept AUX inputs though. In the end I got an aftermarket HU with AUX inputs. Looking back, it was probably not the best solution. I should have ran the factory HU into the AUX inputs of the empeg. I know this means buying an amp, but it is not much more expensive than buying a specialized adapter or an aftermarket HU. It is a slipperly slope though. Once you do the amp, you will want to do more