We use Tom's SSL Redirect Module on a multi portal DNN installation and a "wildcard" SSL Cert ($200 from GoDaddy).  It has the added benefit of securing the login and registration pages (or any other page) on your portal.

The thing to remember is that using it this way will require that your secure domain is a subdomain of a single "central" domain (typically your host or zero portal):

http://www.mystore.com would redirect to https://mystore.myportal.com