There's several ways to do this, but the best IMHO is to add your own domain (the one your are going to transfer later) as a fully hosted domain.
The web directory /home/username/yourdomain.com/ will then be created and you have to upload your content to this folder.
Then create a mirror with the following info:
Domain that mirrors: yourdomain.dreamhosters.com
Domain to mirror: yourdomain.com
http://yourdomain.dreamhosters.com will then show the content of your yourdomain.com-folder.
Hope this makes sense, otherwise feel free to ask again.
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