This is possible.  What you have to do is create your own membership provider.  The easiest way to do this is to create an HttpModule that handles your authentication.  Then, in your web.config replace the DNNMembership section with the one you created.  If you want want this to work for all portals, you will also need to edit the portal users stored procedure to share users across all portals. (I believe there are some 3rd party components out there that do this).

A piece of advice... if you do this, you should write your HttpModule to handle the authentication, but still use the DNN and ASPNET tables for users and roles.  You can have your membership provider authenticate and then insert/update the user into the appropriate user tables.  You will probably want to handle roles the normal way.  If you do it this way, you won't be editing any core code and will find it much easier to upgrade when new versions come along.

hashmipk2000,

1. don't expect many answers with a nonsense subject line

2. writing an individual membership provider might not be the easiest task.

3. please have a look at the latest blog posts by CT member Charles Nurse, he is currently writing a new DotNetNuke membership provider.