Migrate part of a Drupal 6 installation to a new Drupal 7 installation
I might try the Migrate module.  It takes some coding but  but the documentation and example modules for it are very detailed and helpful. Basically you would create a migration class for the users (see BeerUserMigration) and a class for the forum nodes (see BeerNodeMigration). 

One of the great things about this module is it creates a mapping between both data stores which allows you to rollback and re-run the migration until you get it right.