NIF file
NIF Files
NIF files are the meshes of TCS. Currently the options to edit NIF files are limited because no NIF exporter for the Oblivion format has been released by Bethesda. Hopefully they will soon.
The existing NIF files can be used in creating new objects in the TCS. They will have to be extracted from the BSA files first. See the Meshes and Textures category page for extracting details.
With the tool NifSkope you can do some rudimentary modifications on existing NIF files, like retexturing, adding transparency or emmissive color ("glowing" meshes). You can also remove polygon-blocks when the NIF consists of more than one, but you can't change the model's general shape.
Retexturing
See the NIF Retexturing page for details.
Importer/Exporter
See the NIF Importers and Exporters page for exporters for Blender and 3DSMax