I believe you can transfer existing domain name, but it takes a few weeks...
See what they sent me when I first signed up:
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Domain Transfers Want to transfer a domain name to SBI!? Please take my advice -- do not do that right NOW. Instead... Complete the first five DAYS of the SBI! process. That will yield your PERFECT domain name. I can almost guarantee that you WILL find a better domain name. You have nothing to lose by following the first 5 DAYS. Andso much to gain. Of course, if you do NOT find a better domain after DAYS 1-5, transfer your existing one THEN...
Can I transfer a domain that I already own to SBI!?
Yes, you can transfer an existing domain. However, we highly recommend that you carefully consider Ken's sound advice before you make your final decision. A domain name transfer is not recommended for most people because...
1) When you brainstorm, you may very well find a more profitable domain. We know. We see it happen all the time. If you choose the wrong domain name and want to change, there is a substantial charge because it is a manual process.
2) Your existing domain name may not be the perfect one for your new site. If you already know the theme of your site, ask yourself if your domain name reflects that theme well. And does it contain your most important keyword? If your answer is no to either of these questions, you'll be better off registering a new domain name.
3) Transfers often entail tons of practical problems, all of them beyond our control, which cause a great deal of frustration and delays for you. For example...
a) your registrar -- some are easier to deal with than others
b) the security system -- you wouldn't want someone else to be able to transfer your domain to them, would you? Unfortunately, that puts all kinds of steps into the process that make it tricky and difficult. So it could take up to 5 days to transfer a domain if everything is in order, and much longer if things are not.
If you are still sure that you would like to transfer in your domain, please do some important verifications for your domain to make sure that your transfer will proceed without a hitch...
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Hope this info helps