Alternatively, you can open the portrait model in Magos (or convert it to MDL and open it in notepad). Then import your texture (go to Windows -> Texture Manager) under the path of the regular archer skin. Remove the old one (right-click, "remove" or "delete"). Then you would edit the materials to use the proper skin (Windows -> Material Manager, double click the material reference, double click the material object, and set the texture to the texture you imported). That way, you don't need to import the same skin twice.