It seems to me that the newbb in its structure is similar to phpbb so I used the available converter that is attached to the bbpress forum. Because I'm using newbb, I've been trying to modify it by replacing individual table names in a database. I just managed to move categories of forums with descriptions :http://www.kulturystyka.org.pl/test/forums/....
I need professional help modifying the phpBB.php file in order to "pull out" the topics, posts and users of the newbb forum.

http://www.kulturystyka.org.pl/phpBB.zip

For a person who is dealing with php every day is a piece of cake, but for me black magic. I would be very grateful for your help.

I found something like this:
https://bbpress.trac.wordpress.org/browser/trunk/src/includes/admin/converters/Example.php?order=name

Download link:https://bbpress.trac.wordpress.org/export/6392/trunk/src/includes/admin/converters/Example.php

Regards, Michal