As a 91B, can I get 92A as a secondary MOS?

In my humble opinion, to be honest with you. Don't worry about a secondary MOS, a 91B is all the MOS that you need. As a 91B you are expected to know how to run a SAMSE/G-Army box. When you become a Motor SGT, you are expected to know all aspects of what your 92A does on a daily basis. No Chief Warrant Officer wants to hear that work orders didn't get closed, services didn't get updated, man-hours were not logged, parts weren't picked up and processed, key control wasn't straight, ARIMS was updated......etc....just because the 92A was on duty or on leave. As the 91B, we expect you to hop in and perform those roles because guess what, as a Warrant Officer, that's what's expected of us and whole lot more. Go to the SAMSE and G-Army operator an supervisor courses if available and continue to balance your total maintenance and logistics workload and then one day you will be ready to put in that WOCS packet.

Got to go to the school, got to have the training on paper

Track your training and experience. Have a officer under that MOS give you an evaluation for the experience and turn it in to the TRADOC School for that MOS