Arcane University:Niftools/Chunkmerge for Skyrim
The Beyond Skyrim Wiki — Hosted by UESP
ChunkMerge is a tool that can be used to add complex (concave) collision to a NIF. It is packaged with a few other tools as NifUtilsSuite (download). There is a standalone version of ChunkMerge, but this version is out of date and can cause game-crashing bugs.