Last time I checked, the system for add-on and parked domains requires that the nameservers be set to x10hostings in order to add the domain to your account.
Once the domain is added here, you can go back to your registrar and reset the nameservers back to theirs. The IP of your free account is 198.91.81.2
Please note that the cPanel does have zone editor capabilities.
Also, they sometime change the IP's of the servers, which means you will have to notice and then change your own DNS entries if they are kept at your registrar.