I searched a lot, but I found only a very old tutorial thread that had a video, which unfortunately is a dead link.
But I know it must be possible to do so, given the quantity of mods with raw crosshairs talking about such guide, and a few ones already baked in common.lvl
You have edit the common.lvl file in order to create or modify crosshairs, this is however for Battlefront 1.
I dont think it is possible to add ones to BF2 unfortunately, I may be wrong though, since I mod exclusively on BF1...