Nipples can be bought per size or made to fit. Rigid metal conduit is UL approved, you just have to cut to length and make sure it is reamed and threaded properly.
Now using black steel pipe has been a big no-no every place I have ever worked. I have done alot of work on Explosion Proof and just using nipples to get through walls. I don't use continous threads, especially on mobile equipment where vibration can cause couplers/locknuts to walk off of the fitting.
