Not from Ning to bbPress but I have heard of resources for Ning to BuddyPress:
https://buddypress.org/2010/04/helpful-resources-for-ning-users/
I know this thread is 3 years old but what did you end up doing djtonyz? Desperately trying to get the Ning forum/discussions JSON file ANYWHERE. Ideally BuddyPress and/or bbPress of course. Any insight anyone!?
You could try the latest version of the plugin, it is hosted on GitHub here
https://github.com/boonebgorges/Import-from-Ning
Actually it looks like the WP hosted version is also the same version
https://wordpress.org/plugins/import-from-ning/
Thanks Steve! Here’s my latest reply on that same thread: https://wordpress.org/support/topic/anyone-supporting-this-plugin-help-w-discussions?replies=28#post-4951643. Giving up on the migration and sending the client to GROU.PS in the meantime since they will migrate Ning data (including forums). Then start 100% fresh with our beloved bbPress and BuddyPress.