Not with Kontakt Player, which is in essence a demo version for you to assess whether purchasing the full kontakt is worthwhile.
Libraries can be made compatible with the Player version if the producer of the library purchases a license from Native Instruments. But virtually no one making free libraries for the fun of it out will want to pay out so that people using the Player version will be able to fully use it.
I used the Kontakt demo for maybe ten years before I purchased it as part of Komplete 10. It was a demo version that would run any library for 30 minutes and then time out.
I believe with the Player version now you can save any changes you make that will load in the full version of Kontakt (useful, if you do decide to purchase it)
However, if the library comes with wave files, you’ll be looking at DAWs that have some tools or plugins or Standalone tools to help you create your own instruments.
And I think Christian said he was working on something that would allow us to load any (future ?) pianobook libraries in any format ?