Transferring an Existing Domain
If you already have a registered domain name, you can transfer the domain to your store in two steps. First, you must add the URL to your Magento Go account. Then, you must change the DNS settings of your domain to point to the Magento Go server. The steps are essentially the same as for setting up a new domain.
Adding a Custom URL
After these changes have been made, it will take from one to two days for the domain to be fully recognized across the Internet. During this time you can continue to work on your store.
Note: Magento Go does not support Name Server transfers.