Question:
Hi there, how do I add multiple domain names to 1 user account?

Answer:
to let one user have more than one domain, you need to create that user as a reseller.
go to resellers, click on add reseller, and specify maximum limits of your new reseller...

if you created a normal domain/user then decided to let him have more domains, then you may:
go to your phpmyadmin, ehcp database, browse panelusers table, edit the user that you want to increase domain count, set maxdmoains field to what you wish... set other parameters too as you wish...

this way, the user is a reseller from nowon...