I just converted a phpbb2 database to bbpress database using this script:
Convert phpbb2 to bbpress database
It was surprisingly easy to do. The script worked great. I used the script to output the converted database to a file, and then opened the file and edited it to adjust what I needed. The phpbb forums were so full of fake spammer users I didn't convert them.
I also had to edit the bbpress forums to advance the topic_id, forum_id, and post_id's ahead of the ones from phpbb2. That was the easiest way for me to do it, might be different for you.