Its not that hard to add one! Just a couple other wires, it pretty much is plug and play. Took less than a half hour, and most of that was just taking apart the CTS console and putting it back together!
To get a basic functional Nav, you only need to splice in a wire from behind the instrument panel to the VSS terminal location on one connector for the Nav. Phase 2 is optional and adds voice commands and audible directions. I found that having the Nav talk to me was more annoying than anything and disabled it via menus.
If you feel comfortable pulling the IP and center stack apart, the rest is easy. Phase 2 requires you to pull the rear seat, the trunk liners, and the underdash panels, for access to run the wires. Time consuming, but not difficult. It kinda scares the wife, if she sees you tearing apart your new car, though.
Pete was doing the full conversion for a customer. If you don't feel comfortable with doing it yourself, he may hook you up with his installer. Pete's just South of Baltimore.