Importing is simple, you need Maya 6 (a 3D program) and the Nitro 3D Plugin: http://www.dshack.org/page.php?page=dshat (on the bottom)
Sound reasoning but all download play content should be RSA signed. You can edit it all you want but it'll break on the client's DS unless s/he has flashed it. Not that flashing is a big deal but I really don't think many people nowadays are even aware of it.FAST6191 said:@ YoshiInAVoid obviously I can not speak for Gericom but the file types are largely the same (although lots of compression) and most download play files are compatible with standard DS file system parsing methods.
Not to mention if you have made it to the point in life where playing hacked multiplayer mario kart DS is something you want to do and can not sort a flash cart capable (there was a patched version around and I recall making a standalone version) of playing mario kart you are doing something wrong.