This is the scenario: I have a Drupal 6 site, and another website that doesn't use Drupal. This 'other site' is in the same server, and the same hosting account. I need Drupal to be over the other website with his theme, regions, blocks and sessions/roles/authentication. The other website must conserve its own system of session, authentication, content and their URLs (but like already said, the theme, blocks, regions, authentication, must come from Drupal).
Update: iframe is not an alternative (SEO issues).
Update: This guide Integrating third party applications seems useful, but certainly I do not know how to implement it.