Yes, you can host as many domains (and/or sub-domains) as you'd like.
If you already have the domain registered at DreamHost, you just need to add it to the hosting system in the panel at Domains -> Manage Domains -> Add New Domain / Sub-Domain. This will create a new sub-directory in your home directory, which by default will have the same name as the domain (eg: example.com). This directory is where you upload the publicly accessible files for this domain.
If the domain is registered elsewhere, you will need to either transfer the registration to DreamHost, or configure its name servers to the DreamHost name servers (ns1.dreamhost.com etc.).
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