You can switch to our nameservers for a few hours until you can add the domain to cpanel, and then switch right back. With that A record it will work with someone elses nameservers, but cpanel won't let you add the domain unless you're using our nameservers. The reason why is because if that check wasn't in place people could add all sorts of random domains like google.com. The other option is you could use our nameservers and just copy your mx records and everything over to be hosted by us.