Using external app to inject new Drupal users

I'm looking into using Zen Cart for a shopping cart application, but I need to integrate that into an existing Drupal site. Does anyone have experience in creating Drupal user accounts outside the normal registration process? In other words, can someone tell me which tables/fields have to be populated in Drupal to keep Drupal happy? Since Zen Cart requires much more information (addresses, etc) to create a user my assumption is that I will redirect the Drupal registration to the Zen Cart page and add a database insert when saving the Zen Cart user account so that the new Drupal user is created at the same time. So the user, without knowing it, will be creating both accounts at once whether they plan to shop or not.

There are numerous posts here and on the Zen Cart site from people interested in merging these two applications somehow (merging the user info I mean), but nobody seems to have done it yet. If I figure it out I'll post the solution for you guys.