Wouldn’t it be easier to use import script? This importer can be an excellent base to make your own, just change DB table and row names from PHPBB’s to yours, and comment out anything that is not user-related: Migrate a phpBB3 forum to Discourse 
Wouldn’t it be easier to use import script? This importer can be an excellent base to make your own, just change DB table and row names from PHPBB’s to yours, and comment out anything that is not user-related: Migrate a phpBB3 forum to Discourse