If you can find something similar with the same outside diameter that has the same threading but is overall thinner there is absolutely no reason why you can not swap those spacers out. You can set the pre-load just fine with a thinner spacer/nut just make sure it lines up with the cartridge bearing in the same fashion as the ones that came with the hub.

Measure the distance between the inside faces of the fork dropouts and measure the distance between the outside faces of the locknuts on your hub. Subtract the two distances and divide by two to figure out how thick/thin the threaded spacers need to be.

Good luck.

-j