Hello Aqmike,
If the domain name is already registered, then you need to first change the name servers to point to us (or create an A record to point to our hosting server). I'll give you several links at the end of this response to explain the answers I'm giving. If the domain is already pointing to our hosting service then you simply need to add an ADD-ON domain in order to host multiple sites with unique domain names.
Updating Domain Name serversAdding an Add-on domainRegistering a New Domain name with AMP
If you have any further questions, please contact technical support or leave a comment at the bottom of the page.
Regards,
Arnel C.