why would you want to move from a phpBB board to a bbpress board. phpbb has many more features, is much more secure etc. etc.
and there are several bridges to make it work with wp just fine.
I did some research on a convertor. everything I found was not very good. it might convert the users and posts but just about everything else has to be redone manually.
groups, permissions, etc.
I know you want free but if you just have to use bbpress then I think this is your best bet:
https://wordpress.org/plugins/cms2cms-p ... escription