Yes, there are a couple of ways you can do this.
Firstly, you add your domain to the DreamHost system as 'fully hosted', even though the domain is currently hosted and active elsewhere, you just won't be able to access the DreamHost version by it's real domain name.
To access and test the site you can create a free dreamhosters.com sub-domain and set it to mirror the domain you added above. More information can be found in the wiki article below;
Another option for accessing your site before you point the domain at DreamHost is to edit the hosts file on your computer, making the domain name resolve to the IP address of your DreamHost server.
Edit: Damn, beaten yet again by rlparker, I must learn to type faster
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