HaloMods "Classic" Read-only Archive

im doing this mod, and its my first time modeling. whenever i extract a model and edit it in blender, it wont import back into hmt. it'll just say HMT has incountered an illegal operation, blah blah blah. any help? please?

Is the file that you saved your model as also a .obj file, or the problem is that you dont have the right amount of verticies and indicies.