In addition to the phpbb settings file the new version of the converter that I've been working on really needs a complete installation to work properly. For example, it copies avatar files associated with users being migrated into the appropriate SMF location, so it needs the avatar directories to exist and have the relevant files in them. It can also copy Smileys, but you can select the 'SMF only Smileys' option to bypass that.
If all you have is the database, you've got two choices...
1) Hack the converter to make it convert just the data you've got available and want to migrate.
2) Hack your data to remove anything you don't have available. So if you don't have any avatars left, you could change the phpbb_users table to empty out the avatar related fields (and set the avatar_type field appropriately) so that the converter doesn't try to move that data.