[judec]

Well, I'm affraid it's not possible. There is no data standard used for GPS software so every map provider has it's own format.

Even if U'd find a usable map from Navteq which is the map-data provider for BMW, there is only a small chance it could work.

A few years ago, when I wanted to buy a Panasonic nav unit, I asked Navteq (which made the maps for them also) if it is possible to add the map of Hungary to the Panasonic EU DVD, where it was missing from. Their answer was NO as they compile the data for each of their customers separately based on their specific requirements, so that specific map (or folder of data) I'd buy for other unit won't be compatible with the rest of Panasonic stuff.

Anyway - if U happen to find a GPS unit containing your city with data from Navteq, try it. Burn your custom nav DVD with compiled data and try it in your navi, it can't screw up anything (except one wasted DVDR, big deal).