In principle, writing a custom converter wouldn't be that complicated: the stock model/texture converter isn't all that fancy, too, and - except for the issues with modern OS's - works pretty well, right?
The crux will be file formats: in order to make a useful converter, we need to
completely understand the file formats Nexus uses for its textures and models. Actually just for the models - I already reverse-engineered the texture file format before and should have my docs about it somewhere on my HDD.
Yeah, but the models. I know enough about that format to extract a Nexus model, even including animations and stuff... but there are still large sections of the file format that I just don't understand yet. Writing a converter before fully understanding the target file format will be ... pretty hard, I guess.
Maybe someone knows the original programmers, who designed that file format for Mithis, and could persuade them to help us fill in the blanks, if they still remember the details? Or just someone with a talent for reverse-engineering 3d data?