True, there is no UI available in SFC1 that are able to be added. It can be done, but will probably not get added to SFC1. Now in Shipedit, it does nothing for adding UI's it just uses the ones already in the game.
Now if you look at my sig, that is a custom Mirak ship UI in SFC:OP. So as time goes on, some will probably be added to the game, but not under the control of end users.