If present, open the .skins file for the appropriate aircraft, which AFAIK is like the F/A-18A/C, F-15C, and like the Su-25T. The .cmd file has all the texture assignments, and cannot be changed, at least the amount of skins. Granted... nobody's tried with adding textures to the .cmd file, but the general rule that the CLSID is what it "looks" for, and maybe hardcoded into the .exe file.
Verdict? No, you cannot add skins. However, ED plans to allow more skins to be added through the .skins setup for ALL of the aircraft, or the vast majority.