Have you tried NifSkope? I didn't have much luck with it so far, but your mileage may vary.
My problem was that I couldn't edit texture path in the blocks/nodes already present, and when i added new ones that gave the option of choosing texture, which also displayed in NifSkope, the GECK refused to accept the file.
No tutorials I've found were helpful enough, and most are aimed at Oblivion modding.