Allright, so I attempted this and the result was that Davis (the bully who I edited) changed into a complete white character.
I really don't know what I did wrong.
What I did was:
1. Extract GN_Bully05.nft
2. Rename to GN_Bully05.nif
3. Open with nifskope
4. Extract
5. Open with PAINT.NET
6. Did some changes
7. Saved
8. Choosed the file I exported and edited as texture.
9. Embed
10. Removed block
11. Launch game
12.
13. FAILURE!!!!
So yeah....
It's a pain in the ass.