Yes you can. You will just need to do a redirect from the domain you already have to your GuildPortal site. The place your purchased your domain from will need to tell you how to the re-direct using their system.
I'm hoping that this can either be resolved so that we can use our own name or cancel the new account with a refund, either full or a partial reduction for the few weeks we've had the site.
I posted here with the hopes of getting an answer/solution before having to contact support.