I know that Navteq supplies map data, but I don't know if they actually make any data discs or formats for anyone. So you may find for instance that the file format and structure for the Garmin maps is completely different than that of any other product with Navteq data.

It might be like comparing a PNG to a GIF. The hack here will be decoding both formats and then converting the data. Then figuring out how to structure the disc to make the newly created files accessible to your in-car nav. Sounds like it could be pretty complex.