I found out that I need an older version of blender for exporting meshes as .nif files. Installed that and everything that is listed http://niftools.sourceforge.net/wiki/Blender.

It works so far and I can export to nif. My question is what settings do I need to export the meshes with? As mentioned in the above linked page Skyrim is not supported by NifScripts (so no preset available) but there is a workaround. It just doesn't elaborate. Anyone here knows more?

I tried different presets (namely Fallout and Oblivion) but the nodes are wrong. I get f.e. NiTriStrips and Niftools isn't able to convert the data (not even copy & paste the vertex coordinates).