One way to do this would be to write a custom module that does just the extra stuff you want.  You can extend the built-in user profile to include the added parameters.  What I would do is setup the portal (Admin -> User Accounts -> User Settings) so Redirect After Login always sends the user to some page that has your custom module on it.  Your module then extracts whatever profile information you need.  A nice, finishing touch would be to pay attention to what page the user logged in from and send them back to that page after you've retrieved everything you need.

No custom registration page is needed to do this.  You just need a simple module (really no interface is even necessary) to do the processing you desire.