How to migrate from DotnetNuke to Drupal?
There is a very good module to assist migration from other CMSs to Drupal called the Migrate module.


  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, files, terms, and comments are included - it can easily be extended for migrating other kinds of content. Content is imported and rolled back using a bundled web interface (Migrate UI module) or included Drush commands (strongly recommended).


Link : Documentation.