If you have a domain for which you manage the DNS on servers other than i7MEDIA but you want the domain to be used on a website hosted by i7MEDIA, you still need to add the domain to your hosting account so it can be attached to the website. To do this, follow the instructions in the "Creating a Domain Alias" or "Managing Domains" knowledge base articles. When you follow the instructions in either of these articles, you do not need to "Enable DNS" as the DNS isn't on i7MEDIA servers.
Please note that your hosting account must have available domains to assign one to a website. Adding a domain to a website doesn't alter the DNS of domain if the domain's DNS is not hosted by i7MEDIA. Nameservers and other registration information for the domain aren't affected by the web hosting control panel at all.