If you are running a post 3.6.0 openfire, administrators are now stored in the database. You can also add admins from the admin console. Simply go to the user’s details and click the box to make them an admin.
You could go to the “System Properties” and change the “admin.authorizedJIDs” value (Comma separated). I suspect you would have to restart openfire after changing that though, but am not sure.