If you're asking help on just the conversion part, here is exactly what you can do, it's pretty simple.
- Make sure the image is 2048x2048.
- Using your photoshop, save the image as a 1 layered jpg or png (%100 rendered or 32-bit quality).
- Open the image with ImageToPAA.exe file. Usually this is part of a bigger texture/ imaging tool system, but this should do the trick for you now.
Download for ImageToPAA.exe (Will remove it soon as you download it):
https://dl.dropboxusercontent.com/u/70799781/ImageToPAA.exe?dl=1 Removed.
About using that texture in addons or missions, sorry but I will not be able to help. I can help you retexture vehicles by walking you through the whole thing though. Problem is uniforms are more complicated (at least for me) since they're also items, and they have more details and stuff, so good luck!