Migrate user-data from a non-drupal site
Take a look at the Migrate module.  From the project page:


  The migrate module provides a flexible framework for migrating content
  into Drupal from other sources (e.g., when converting a web site from
  another CMS to Drupal). Out-of-the-box, support for creating core
  Drupal objects such as nodes, users, and comments is included - it can
  easily be extended for migrating other kinds of content. Content is
  imported and rolled back using Drush commands.