Imd files are always exported with a texture section. However, when converting the imd to nsbmd with g3dcvtr, there are many command-line options, and one of them is to ignore the textures, resulting in textureless models like those in Pokémon maps. This is what you're probably looking for.
And yes, there are lots of overworlds. I'd recommend you to look for a tool with btx reading support, like Console Tool by Low Lines. You can open a narc and scroll through the files. It has a preview window, so it may be useful
I have a question: can I edit edit nsbmd files converted in obj without the texture files?
Because I can't export the obj model into imd without the textures files...
And I got an another question : I have decompressed mmodel.narc in pokemon Diamond and I found btx files that contained Overworlds but there're almost 300! I saw that you changed the hero overworld in Pokemon Dark Platinum, so Which one is the Hero?