There are ways you can do this. I don't know how the reults will look though. I think there are flat clears out there. Since they don't use them in the automotive paint world I can't tell you for sure.
However, if you have your paint mixed at an automotive paint store that sells PPG paint, ask for a few ounces of DX265 Flattening Agent. A little added to the paint will go far. For example, 2 ounces in an RTS quart of black makes for some FLAT black paint.
Good luck and be sure to test spray first. Add a bit at a time.