How to Add 2nd Domain Using STANDARD Email Hosting?

How to Add 2nd Domain Using STANDARD Email Hosting?

My client wants to set up 2 domains for email hosting. He purchased STANDARD Email Hosting--which allows you to have multiple domains.

My client already owns both domain names--through register.com.

I've setup 1 domain name for my client: lizbockelman.com

However, every time I try to set up a 2nd domain name, graphocity.com, I get the message that "domain already exists".
RIGHT, my client already owns that domain name.

QUESTION: How do I add a 2nd domain name where we already own that domain name?