You don't need to transfer the domain registrations, simply configuring the DNS to the DreamHost DNS servers will suffice.
Unfortunately, there is no way to automate this process, the email addresses will need to be manually configured using the DreamHost admin panel. If you have a lot of email accounts to configure, this could be a rather long process.
When you transfer a domain registration to DreamHost, you will need to extend it by 12 months, which will cost $9.95 per domain.
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