Migrating content to a Drupal site is usually a painless experience because Drupal is quite well structured. Even if we were to move the content from non-Drupal database to Drupal database without the help of any API or migrating services, all that is required is to understand the database structures of both sites and then write a series of SQL queries that copies the content from any non-Drupal database to Drupal database; which can be done by any above average Drupal developer.