I'm not quite sure how to answer your question without more details. If you want to use FusionAuth as an external directory, you could encrypt sensitive data and store it on the user or registration objects. While there is built in support for managing keys you'd have to build the actual encryption logic yourself; you can't mark a user field to be 'extra encrypted' in FusionAuth at this time.
That might be an interesting feature; feel free to file a feature request with more details.