[Roger Coathup]

To get the activity in to the BuddyPress activity stream, you use the function bp_activity_add()

I don’t know phpBB, but suspect there will be a hook when threads / topics are saved… That is wheee you would add your code.

This thread points to a similar solution for simple:press.. Hopefully, you can adapt it easily for phpBB: https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/adding-a-activity-to-the-activity-stream/