Query D6 db from D7 module
If you are looking to Migrate D6 Content (Nodes) to D7 system then you can use Drupal-to-Drupal data migration module.

Project page says


  migrate_d2d provides a framework based on the Migrate API for
  migrating content from Drupal 5, Drupal 6, or Drupal 7 sites into the
  Drupal 7 site where it is installed.