The best thing to do is to register with the name you want, and then log back in with the Admin username and change the new user you just created to an Administrator.

After you have logged back in as Admin, goto your Members page.
In the right hand column will be some icons, click on the icon to edit the profile of the username you just registered.
On the bottom right hand side of the page that follows will be a dropdown box. Administrator is one of the options in that box.

You're welcome