Will I be able to install a sub without replacing the head unit in a 2007 lincoln MKZ?

So I recently bought a 2007 Lincoln MKZ and I want to transfer my old system into this car.

The car came with a THX certified sound system and has a built in amp to power the speakers. So removing the head unit will mess up the entire system.

So can I keep the head unit (which I'm sure has plenty of power) and still be able to install the sub and amp? I'm planning on getting it professionally installed so this has to be possible. I know I need a line converter of some sort. But that's it.
Am I wrong? If someone could please give me some insight that would be awesome.

Thanks!