When you log into your domain panel you should see the 'Domains' tab on the left hand menu. In there you should be able to add domains or subdomains to your account (depending on how much your package allows for ofcourse).
I have 2 fully hosted domains (different sites for each) and one that is mirroring one of my other sites. Pretty quick and painless to do, but remember you will have to change the DNS for the new domain name, so make sure you can do that (without much hassle) with your current registrar.
Hope it all works out for ya!