[@netweb Keymaster 8 years, 3 months ago]

Checkout the phpBB converter, it’s one of the best example implementations.

Use _bbp_old_topic_id to grab your old topic id:
https://bbpress.trac.wordpress.org/browser/trunk/src/includes/admin/converters/phpBB.php#L183

p.s. The above phpBB converter uses bbPress 2.6 alpha which has many new refinements for the importer over bbPress 2.5, I’d suggest you use bbPress 2.6 alpha for any imports due to those improvements, it can be found here https://bbpress.org/download (bbPress.org and wordpress.org both use bbPress 2.6)