It's possible to create new units for the campaign; I recently made a Japanese flying boat as an air unit for a Pacific campaign (if we get around to it).
http://rapidshare.de/files/5001669/H...g_Boat.7z.html
I'm also working on an American cruiser. Someone successfully made a playable British submarine, so it should be fairly easy to model the playable US, British, and Dutch subs that fought against Japan. We'd need to do some 3d editing and, obviously, changes to uniforms, medals, and interface screens.