Here's the simplest answer: YES. :)

And really, it's that simple. You can actually go old-HU to empeg or empeg to old-HU. In the first case you need pre-outs on the HU and in the second case you need AUX-in on the HU.

If you are going to do HU to empeg, then I suggest mounting the empeg in your dash and hide the old head unit - or mount it in an alternate location. The more important display will be the empeg, so you don't want it hidden. People have all sorts of creative ways of mounting additional heads, so I'm sure you'll get some feedback with specifics. Your local car audio shop would more than likely also have some ideas.

Bruno