How can I export users, forums and comments to another Drupal site? I need to move Forums containers and topics and their associated comments and users to another Drupal site.  I am exporting it into an already existing site.  How can I do this?
If you want to export into a live site where there already exists forums, topics, nodes, comment users that you not want to overwrite, I suggest you check out the 
Drupal-to-Drupal data migration module.  It will let you move the data and refactor it in the process.