You can open them via Load/Save option, add them to your Database or use the Quick-Load feature or drag and drop but you can’t add it to the Libraries tab.
Only libraries that are licensed by NI can be added as a library. And as the license is not free, most free libraries don’t have a license and cannot be added as easily to the Libary folder as those that do have a license.
Only the full version of Kontakt 6 will load all the different Kontakt libraries found on Pianobook without limitation. Kontakt Player 6 will run these libraries with a limitation of 15 minutes use. And if you only have Kontakt 5 version, you will only be able to load those libraries made in Kontakt 5.
Many of the libraries on Pianobook come with wav or aif samples that you can use to make your own instruments from using a variety of software.