Are your different sites all part of the same domain (ie are they like xxx.domainname.com, yyyy.domainname.com). If yes, then you can purchase a wildcard SSL certificate and apply it to your website... If they are unique domains, you'd have to purchase SSL certs for the domain in question (this might be different for shared hosting, I'm familiar only with hosting sites on my own boxes)....
With regards to how DNN handles it, once you've turned on SSL (via Host Page), you can set a specific page to be SSL enabled or not. So you do have control at the page level. However note that you better setup the cert correctly in IIS first as if you click the enable SSL and your server is not setup with it, you'll get a page not found error....
Sanjay