sure, you can.
Just log into the dreamshost panel, create the sub-domain that you want (if you heavn't allready) through Domains > Add. Then go and do it again... "add free DreamHost (like say domain1.dreamhosters.com) sub-domains to your account (again through "Add Domain") and set them to "mirror" your actual domains you haven't switched over yet."
This was as soon as the Dreamhost servers get it set up you can access your domain to view it. You should have gotten an E-mail when you signed up saying how to connect via FTP to up-load new files before the DNS changed over, and that method should work the same way to access sub-domains. (something like machine.dreamhost.com)
You might also read this K-base article, it's where I got the quote from. https://panel.dreamhost.com/kbase/index.cgi?area=2824