You are right. If there's similar registry number, you won't get "Add aircraft" page.
Try this:
http://www.airport-data.com/aircraft/?????.html
where ????? is the aircraft registry you want to add. For example, if you want to add N9665, the URL should look like:
http://www.airport-data.com/aircraft/N9665.html
This will bring you direct to add aircraft page, if the registry does not exist.