How to import external database table into Drupal database table?
You have at least 2 choices, none of which is easy in my opinion.


Import the non-drupal-users into drupal using a tool such as Migrate to actually map the new fields into drupal fields, you can use hook_schema_alter() and hook_entity_propery_info_alter() to tell Drupal about these new fields that Migrate is moving over.
If you dont merge these tables permenantly into 1 drupal "users" table you're kinda making more trouble for yourself. You could implement your own authentication method to do an SQL query against your (non-drupal) Users table and if the visitor is found there create a new Drupal user account for them. To do so you'd basically create your own login logic as loosely described here for an external source, Blog Tutorial