Email associated with a domain is handled as specified by the domain’s MX records, which are part of the DNS set-up for the domain. As such, they need to be set by whoever controls the domains DNS.
I do not know of a way to provide email services using Dreamhost for domains that are not under their control. You might consider investigating Google’s Gmail for Domains, which will allow you to point the domain’s MX records at Gmail
To get an authoritative answer, you should pose this question directly to Dreamhost Support via a support ticket sent from the Control Panel.