The contacts in the car are manually entered by the user. There are up to 20 (I think) voice dial slots and 6 of those can be made radio button slots. You can't 'sync' your phone's data with the car (I have non-Nav, but I imagine it's the same with Nav).
Best advice is to set the 20 most called numbers on your car. If you need to call someone else, you'll have to either press a button on your handheld to active the phone's voice dial feature (if available) or have a passenger dial the phone for you or pull over to the side of the road, put the car in park and then dial the numbers into the cell phone's handset.

 

I have Nav, and for that the answer is 'it depends on the phone'. My old Motorola was able to transfer contacts one at a time to the Nav phonebook (but not the whole contact list at once), but my iPhone doesn't appear to have the ability to even transfer one at a time.

 

I did transfer my contacts from LG Verizon to TCH (with Navi) few years ago, but can't remember how to now. All I remember is that it is possible, that it is not easy (I got some instructions then, but somehow I kept missing some steps, until finally it worked). Just got new phone and I won't even try to do it anymore. Too many numbers, many of them I rarely use, so it's much easier to input manually the most likely numbers I may call, like wife, brother parents, closest friends. Frankly, with TCH screen locked out when driving(I'm sick of inputting the sequence to unlock it every time I start driving), voice commands being hit or miss, far to reach dashboard and multiple key presses to access dialer etc. I use phone 99% to dial out regardless, even if I'm parked, or screen unlocked, so then what's the purpose?