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.

1. 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.
   
   
2. 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.
   
   Include "/usr/local/apache/conf/userdata/std/2_4/account/domain.com/*.conf"
   
   
3. Create the folders for the include line, and add a file of any name with a `.conf` suffix, for example:
   
   /usr/local/apache/conf/userdata/std/2_4/account/domain.com.com/custom.conf
   
   
4. Edit the new file and insert the following lines, formatted for the proper domain:
   
   ServerName domain.com
   ServerAlias aliasdomain.com www.aliasdomain.com
   
   
5. Restart APACHE (command line - server httpd restart)