Right now, we don't have the ability to import connections from CB, only from JomSocial. Can you let me know what Facebook integration you were using with CB, and we can investigate further?
The best, immediate, recommendation is to enable the "Automatically Map Users by Email" setting which, if the user connects through Facebook and already has a Joomla account with the same email address, will link them immediately without any questions or forms. It's not as perfect as importing the connections, because if the email addresses are different between accounts, they may have to manually re-link them, but hopefully that will work for now.
Thanks,
Alex