Yes, Ning Data Can be migrated to Joomla.
I have succesfully done Ning Data migration successfully for my one of project.
What you can do:
1. Create a similar layout to that of Ning site using the JOMSOCIAL
2. Import all users of your NING community.
3. Import users profiles.
4. Import member album & photos.
5. Import groups and group members.
6. Import Forum topics and comments.
7. Import groups comments & topic comments.
8. User activites