So I converted an armor from a mod (.NIF) to an .OBJ file and opened it with ZBrush 4 to customize it a little. After that I saved it as a .NIF file in NIFskope and put it back in the mesh folder. Nothing was changed ingame (dunno if I should've equipped a fresh one that hadn't been in my inventory already) and in the G.E.C.K. the preview of the model showed an exclamation mark. So any pointers on what I'm doing wrong here?

From your description... lots of things.

First off... OBJs are incredibly simple mesh files. Converting to them strips out a lot of the necessary data, such as skin and dismemberment partitions. Converting them back makes for a nonfunctional NIF.

Could you tell me what exporting and importing method I should use then?

You need to export it back into Blender or 3ds Max (whichever you use), re-rig it, re-apply the dismemberment system, and then export out of that, and then finish setup with NIFSkope.

Alright. Thank you very much