Yes it's possible. Why don't you start with editing existing classes, it's way easier. Do you have the BF2 Mod Tools? If not go to lucasfiles. I'm not 100% sure if I know how to do everything right. I started with BF1 because it's simpler. I was able to edit classes on BF1. Wait, I just looked at my BF2 mod tools. Looks like they have a visual munger in this version. Should be no problem munging sides. How much about modding do you know?
Munging means compiling
Sides is the directory units and turrets are in.