How to insert user registration data to non-drupal table
This looks similar to what happens when someone tries to access CiviCRM tables from Drupal when the CiviCRM tables exist in different database containers.  

https://www.drupal.org/node/1569656. 

Are these tables in the same database container or just on the same database server?  

You may want to look at How to use separate databases for Drupal tables?

It might also be worth looking at http://wiki.civicrm.org/confluence/display/CRMDOC44/Creating+a+Drupal+user+for+every+CiviCRM+contact

That gives both a manual process of generating Drupal users from a CSV export as well as the drush method.