I'm afraid there is no easy way to migrate data in to here from there. First you have to set up a bot to craw the old site, which extracts the critical data, Title, Cat, thread and post id, time, date, member an content blar blar. That is passed back to a text file, which passed to another script which packages up the data into an sql dump on the fly, that injects the data into a phpbb3 sql database.
There are three package which need to be processes in that way members less passwords, categories etc. and finally the actual post contents. Once that's process has been completed you have major parsing issues within the migrated data. Which means lots of posts are poorly formatted, smiles are messed up and no poll data can be migrated. There are a few other issues. The only parsing tool I could find is for an older version of BB3 and is in fact quite old.
Once the data is in BB3 it needs to be exported and migrated into the Joomla Kunena data sql database and again some elements are lost in translation. The parsing errors are also still effecting the BB3 data so you're punching poor data into a nice new clean database.
Once all that's done, you need to sort the users out by giving each member a new password unique password, which has been posted into the data base before the members try to use it.
Oh and the forumotion guy check for the bot traffic which is crawling and extracting data. Too many hits to quick for example, and they block the bot and you're back to square one. They also move the goal posts with the javaacookie which you need to use as a hook to start the trawling.
I have all the software here to do it and I have done tests with Novums test site over there and imported that into a local install of BB3. It works your admin password gets splatted, so you need to run an sql command to inject a new admin user, so that you can manually get into fix the original admin account PW. There wasn't enough complex data in my test data to do full tests on the parsing and migration to a joomla DB with Kunena. If you fancy taking a crack at it I can provide everything you need.
The tools may need to be tweaked so that they can ignore any string value equal to snook* 'n you will be good to go.