Devil and I had the same problem, which is Paint Shop Pro will not save a bmp in 32-bit format even with the current version. Luckily I have an old copy of Photoshop Elements that I got free with a graphics tablet and that does do the job. Before submitting a skin I just load the diffuse and normal bmps into Photoshop and re-save them with it. The program then brings up a sub-menu that lets me change the bit-rate to 32-bit.
Unfortunately Photoshop isn't free. There may well be a free program out there that can save a bmp in 32-bit format but I don't know for sure. Anyone else know of one?