By default, you cannot add an addon or a parked domain on our shared servers without changing the nameservers first.
However, if you want to develop or migrate your website to us with no downtime, the nameservers change is not an option.
In such cases, please get in touch with our Hosting Support department via Live Chat or Ticket, and we either manually add the domains for you or disable the server rule to make the adding possible on your side.