Suggest you view the Navigation System thread.
In a nutshell, at the moment, there is no easy way to do that but some have tried it to varying degrees. Apparently there may be a very expensive way to mod the unit by sending it to someone for a mod which costa a small fortune. Others probably will come out with a reasonably priced aftermarket adaptor.
BTW, the NAV REC unit used in DC products this year is by Alpine and is one of the better units I have seen.