Are you sure you can not specify the name servers? It seems very strange for a registrar not to allow this.
You can obtain the required IP addresses from within the DreamHost panel after adding a domain to the hosting system, but you should be aware that these IP addresses can change at any time, so you will need to constantly monitor this and, when required, change the IP addresses configured at your registrar.
Another option would be to transfer the domain registrations to DreamHost, this can be done via the admin panel and is a fairly painless procedure.
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