You have to buy a converter of some type ive heard to trick it into thinking its a multi-cd Unit . As for fitting a Nav+ unit bit of a hassle Need gps and mic fitting also etc .
A combined GPS/GSM aerial with the correct connectors is available on eBAY for about £30 and can sit on top of the dashboard.
Microphone from Maplins for about £5 can be fitted in the front interior light housing (the wiring for it may already be there) or fitted in, or on the dashboard somwhere.