I have not seen this done, I've had alook at the phpBB forum and it would be possible to do it would just require someone with very advanced mysql skills. How the SS forum works is slightly strange as it does not have a 'thread' per-se but a series of linked posts so you would need to get that structure correct.
Porting members across would be the first step then the posts. Members would be complex enough as you would have to map the columns from phpbb to the ones in the SS Member table and also add the member id generated to the group 'Forum Members' in the Group_Members table in SS