Yes, you need to add a brand new domain zzz.com (that uses dreamhost nameservers, thereby allowing dreamhost control for the DNS), set as fully hosted and add the dedicated IP to that domain, then you can use a CNAME record to point there.
You can also do this without the extra expense of an additional domain and dedicated IP , but the con is breakage until manual update should the IP change (doesn't happen often). To do that:
[*]Add your domain XXX.com as fully hosted on dreamhost.
[*]Wait 15-20 minutes until the "clock" icon disappears when you refresh the "manage domains" page in the panel.
[*]Click the "DNS" link immediately under the domain name on the "Manage domains" page in the panel.
[*]On the page that loads,scroll down and look at the "Non-editable DreamHost DNS records for XXX.COM"
[*]The very first record listed should be "Type A" with an IP address listed for it's "value"-- copy or write down this IP address.
[*]Ask your DNS provider to add an "A-record" for XXX.com with this IP as it's value.
Using this method, should the IP ever change your site will break until you repeat the above procedure to obtain the new IP and ask your DNS provider to update the A-record. In general, the IP will remain pretty stable--if you make changes in the panel on the manage domains page it might change, if dreamhost moves you to a different server or there is a hardware problem it most likely will change, but of course there is always the case it could change when you least expect it to, for example dreamhost has to reconfigure due to heavy traffic on a site (yours or someone else's) or it changes as a result to thwart a DDOS attack directed at the dreamhost network.)
The best method is the first using a CNAME record.