Hi,
If your phpbb forum is already integrated with Joomla via jfusion, rokbridge, or another of the Joomla<=>phpBB bridging solutions out there, then migrating to Ninjaboard is as simple as using the migration tool on the Ninjaboard Tools page.
However, if your phpBB forum is a standalone one, then there are a few extra steps you need to take.
Luckily, Steven Zeegers over at Joomla and More has written a handy little guide on how to go about the migration.
Find it here.
Forum