No, it's not. It's much easier to go aftermarket and get more features. (Don't get me wrong, I have factory nav and it is better in some ways than aftermarket. I just wouldn't go to this trouble to add it after the fact.)
Things you need at a minimum to add factory navigation are:
Nav head unit
Nav DVD drive unit (this is needed for the headunit to work at all)
Sub-woofer amp
Four channel amp (goes in trunk, the nav headunit can't drive any speakers without it)
Multiple wiring harnesses that run from the front to the back and to each door.
GPS antenna
If you want voice control (factory systems have this) add:
Auto dimming mirror with factory microphone (used only on nav equipped cars)
If you want the steering wheel buttons to be labeled and work correctly add:
nav option steering wheel button assembly
If you want the full THX sound add:
THX door speakers (4) and subwoofers (2)
Advantages to factory nav (compared to most aftermarket):
Fast boot up
Always knows where car is pointed even if not moving, or moving slowly
Even knows where you are when the sat signal is lost
Easy to use interface with big touch buttons, few menu levels to traverse
Advantages to aftermarket nav:
Works with iPODs, MP3s, USB, phones, ...
Much easier to install
Cheaper for newer maps
Some can be customized