About the only way you'll be able to separate the models is by exporting the 3do to either .3ds or .obj and importing it into the 3d modeling program of your choice. I know in 3ds Max, you can kinda separate the models by "element" in the edit poly mode as long as the vertices are properly welded for each make. It takes a few minutes, but it works. I would assume that not all mods would be as easy to separate like that, but it's worth a shot.
Sounds like you are on the assumption that you can edit a few things and recompile it to make a mod. Without the original scripts from from when the mod was created you are wasting your time.