Hello, here's my problem

I have Overhaul 3 installed. So I got bored and wanted to experiment a bit and tweak beter bodies parts in Max 4, a tail for instance, so I take for example khaj_f_body_clean.nif which contains the whole body except for the head of course and edit the tail's form by moving the vertices. I don't even add new vertices or anything cause I know I can't do animations yet. So I get a new looking tail. Then I import Xbase_animKnA.kf to see how the animations work and they work perfectly, just as the original model with a new tail. BUT I cant get it back to its nif format, I tried to export in many ways but it just won't work. The whole body wont appear in CS/the game, and if I import the exported nif back to Max the model is broken and if I try to import anim file over it I'm getting a complete mess of the mesh looking like some flying macaroni monster. Also I tried to import and export the original unedited file back and its the same problem so I suppose it is completely in bad exporting of the model and not because I messed up something.

I can create statics or head/hair just fine and they work in game but I have no experience on working with creatures.