In order to migrate an existing SMF Forum to Drupal you need to first migrate to phpBB as there is no direct migrate tool at present.

To migrate see this support forum entry:

http://drupal.org/node/54677

Here are the direct links to each tool required to make the migration:

1. converting your SMF forum to PHPBB ( http://www.google.com/search?q=smf+site%3Aphpbb.com )

2. converting your PHPBB forum to Drupal ( http://drupal.org/project/phpbb2drupal )