Hi Simon and welcome to the forum.
Luckily, this is the default behavior for all new domains or sub-domains added to the DreamHost system.
This is easily achieved at DreamHost you just need to add the sub-domains via the panel. Go to Domains -> Manage Domains, click on Add New Domain / Sub-Domain, fill in the relevant details in the Fully Hosted section then click the Fully host this domain now! button. Note: you need to enter the full sub-domain (eg: subdomain.mydomain.com) in the Domain to host: field.
I should note that the DreamHost way of handling sub-domains is somewhat different to the cPanel way. At DreamHost sub-domains are treated just like regular domains and a new web accessible directory is created in your home directory for each sub-domain, just like regular domains. These directories will, by default, have the same name as the sub-domain.
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