Hm... really? I'm actually typing up a guide on how to edit the maps in generation 5. And for hacking generation 4 and 5 just requires more tools more than anything, and maybe some hex editing.
There's also a tutorial on map editing in generation 4. But I can explain the importing part, which is that once you've exported the model as .imd from either 3ds Max 6 or Maya 7, you can then use the command prompt and then go to the folder with the program g3dcvtr.exe, then type "G3dcvtr.exe [model1].imd [model2].imd -o [outputname].nsbmd". And then import it into the ROM using Spiky's DS Map Editor.
But if you don't feel like doing it, that's ok. ^^ You would be used to hacking generation 3 by now, generation 3 uses one whole binary file as a ROM, where as the NDS ROMs have smaller files separated by a named headers such as "BMD0" or something, which indicate it's file type binary. But having said that, it feels more easier to me do hack NDS ROMs than GBA, but to you... it may feel like a totally different experience, like how it is for me hacking 3rd gen. It's quite hard to get used to a different platform where it's programmed differently from the you're already on once you're already used to the one you're currently on, which I totally understand of course. 'Cause I find hacking 3rd gen quite a nightmare. lol Maybe in the future I might continue my old ROM Hack on the GBA after LavaRuby. ^^
My goal is to make 4th and 5th generation hacking as common as it is with generation 3 hacks. And to do that... the map editing guides will be written, then with my ROM Hack Pokémon LavaRuby and Team Fail's Pokémon AquaSapphire will encourage more 4th and 5th generation hacks.