Don't know of a DNN integrated way to do this but you can programatically through the SQL server database or other custom modules.

Knowing a userid from portal A, you would insert a new record into the UserPortals table with this userid value and the portalid of portal B and set authorised to true. Then you must figure out the roleid values in the Roles table where portalid is B that you want to add the user to. Then do an insert to the UserRoles table for each role you want this user to have from the roles table with the roleid for each role and the userid.

Other than that, I know you can create your own module to do this or you could buy a user management module that can do this from the DNN Marketplace or Snowcovered. If you are not a programmer who could write a module, but are technically savvy with some sql and html ability, you could also use something like ListX by BI4CE to create your own module to do this.

I used to do AD integration back in version 3.x and while not robust, got the job done for usernames and passwords, but that was the entire level of integration. These modules were typically on Snowcovered, and I used one from there. Today in DNN some level of AD integration exists and is on the Admin>Authentication tab, but I believe this is still some sort of synch process between AD and the Users and aspnet membership tables. Not sure though. I think I even saw a new AD integration on Snowcovered recently so maybe it has some enhanced AD features.

Anyway, I guess I'm saying I think it can be done, check what DNN now provides and see what's in the marketplaces haveand learn the differences to find your best solution.