Just to clarify what khatfield is suggesting, you would need to add a CNAME DNS record to achieve this.
For example, if your dyndns hostname is somename.dyndns.org and you want to direct the somename.somedomain.com to that hostname (and indirectly to your home IP address), you would do the following;
Firstly, do not add the somename.somedomain.com sub-domain to the DreamHost hosting system.
Go to Domains -> Manage Domains in the panel, click the small [DNS] link next to somedomain.com then add a DNS record with the following settings;
Now you just need a dyndns client on your system to update the dyndns.org hostname whenever your home IP address changes.
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