how do I add Greek and Hebrew fonts
Download and install them. It’s something you do on the level of the operating system. Restart LibreOffice to access the new fonts - the fonts list is only loaded when you start Libre.
Come to think of it, many Unicode fonts have a full set of Greek and Hebrew characters, you just need tools to access them from the keyboard.
For Hebrew, you may need to enable CTL languages: Tools - Options - Languages → Show UI elements for bidirectional writing, and select a default font/language just above that.