you’ll have to set up a mirrored domain to accomplish this. (a search should have revealed this) BAsically you set up a mirror of your domain here at dreamhost from whatever.dreamhosters.com. It may be a bit confusing, but it really works to accomplish what you want. It’s not a mirror of domain.com, but it’s a mirror of what dreamhost has for domain.com.
There’s also a sigguestion you could vote for (Panel > Home > sigguestions) asking that dreamhost automatically set this up for you when you create an account.