You should go to the dreamhost panel and set up a new domain witht he same name as the one you want to move over.
Next, if you're already got a domain set up with Dreamhost, set up a subdomain of the existing dreamhost domain as a "mirror" - this is all available in the "Manage Domains" section of http://panel.dreamhost.com.
If you've not got a domain with Dreamhost, then you can edit the "full hosting" of your .dreamhosters.com and change it to be a mirror of the domain you want to move over. - You should have already set up the domain as per the instructions above.
After this is done (and allowing for DNS updates etc), you will access your domain with the subdomain you created. When it's all confirmed working, get the nameservers updated on the "final" domain to point to dreamhost.
You'll need to make a few little tweaks to the domain to make sure you're using the right domain path for when it moves over.... basically change any reference to the subdomain to be referencing the correct domain directly.
It's hard to put into words, so i hope that makes sense!
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