Yes. This is known as a SSL wildcard certificate. These certificates are available through a number of providers, including Comodo. We don't currently offer them through the DreamHost Panel, but if you've purchased one elsewhere, you can certainly use it with us — just enter it as a "manual configuration" certificate for each of the domains you want to use it with.
Keep in mind that they're vastly more expensive than normal SSL certificates. (The price through Comodo is over $300.) Unless you have a lot of subdomains, it may well be more economical to just buy a bunch of normal SSL certificates through us!
Yes. SSL certificates don't care who's hosting the server — all they care about is the domain name. However, you do need to have control over the "parent" domain to purchase a wildcard certificate. (So, in this case, you couldn't purchase a certificate for *.dreamhost.com, because you don't control dreamhost.com.)
Also, if you have a complex system of subdomains, please note that wildcards only cover a single level of subdomains. A wildcard for "*.example.com" would cover "blah.example.com", but not "blah.blah.example.com", for instance.