I would not try bringing in topsoil. An acre plow layer (1 acre of soil 6 inches thick) weighs about 2,000,000 lbs. You would need to add over 60 tons of topsoil to have an acre plow layer on your thin soil area.

I think you may have more success planting a crop in the fall. I would try planting oats in the fall before a good rain. Plants growing in cool fall soil use less water, and you can plant them a little thinner too to try to save more water.