You can definitely use an existing domain with Site Builder!
You will just need to adjust your A-Record and CNAME with your domain's registrar. To update your domain, please use this information:
CNAME: WWW websites.reverbnation.com
Once complete, you will want to claim your domain through ReverbNation by going to:
Tools>Site Builder>Settings>Domains and search for your domain.
You will see that your domain is already registered but you can still claim it. Claim the correct domain and you should be good to go! Note that it can take up to 72 hours for your domain to start pointing to your ReverbNation website.
If you are unsure who your registrar is, create a ticket and we will be glad to help you find that information.