Just be careful. Dreamhost doesn't like (and disallows) people creating multiple accounts and transferring the hosting of domains between them because these schemes had previously been used by people to roll hosting from one promotional account to another and gain virtually free hosting from them in perpetuity. This is obviously a bad thing for them and is specifically disallowed in the T&C of our accounts with them.
Your case could be materially different because of it just involves the registration of a domain and not the hosting of one.
Then again, the intent of promo codes is to provide a marketing incentive for non-customers to become customers. Once you've already become a customer (however small), you're viewed as "hooked" and generally not eligible for new customer promotions.
This is just my reading of their T&C's, but don't be surprised by the outcome of your inquiry (but I do hope for the best for you!).
What the other person advised may be possible, but you may run into trouble with the transfer because you might be considered to have two open accounts with them and transferring a domain between them.
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