It's been awhile since I've been in the Army Guard but based on a fairly recent conversation I had with a couple of current Guardsmen who went in the Guard as helicopter pilots with no fixed wing military MOS, but with fixed wing time,I'd say you'd probably have to go in as a rotary wing aviator and then wangle a fixed wing slot as these folks had to do. They got a Shorts,I think the Army calls it a Sherpa, slot. It goes without saying to check with the various units in your area.
One had limited fixed wing vis a vis the Naval Air Training Command, the other had thousands of hours in the Shorts as a commuter pilot, but no military fixed wing time.
The up side is you get some decent financial help as a part timer, the downside is you may have to rely on your CFI to build fixed wing time. That's what I had to do, but CFIing taught me a whole lot about civilian aviation.