new areas of Bahria town are not yet included in the GPS car maps .. however, Google maps functions better. Once I was in Bahria Town and my car navigation showed open area ... while the GPS on Nokia N8 gave complete map.
I am not sure if it is possible to mark a point which is not there on the maps for car navigation. Shalimar Town in Gujranwala is a new area and not yet on the maps of naqsha.net ... I tried to save the position of a relative's house and it gave weird directions next time/
However, it is possible to save new locations on phone Google maps.