The cPanel interface for adding domain aliases (aka parked domains) will only work if the domain being applied has the nameservers pointed to a nameserver recognized by the same cpanel installation. These instructions can be followed to park a domain alias on a WHM server account where only an A record exists on the domain.
Add DNS Zone in WHM - from the WHM console use the Add DNS Zone wizard to create the new Zone on the server, attached to the account in question.
Edit `httpd.conf` file to allow includes - from the console as root, edit the `httpd.conf` file for your apache server (ie /usr/local/apache/conf/httpd.conf). Look for the `VirtualHost` block where the domain is configured. Toward the end of the block is an `Include` line that is commented out. Remove the comment mark.
Create the folders for the include line, and add a file of any name with a `.conf` suffix, for example: