The library manager is for manipulating the libraries, not for manipulating the contents inside the libraries.
“Save as” allows you to move one item between Kicad libraries while changing the name.
“Copy/Paste” allows you to move one or more items between Kicad libraries but not change the name (name changing must be done independently).
“Import” allows you to add items to a Kicad library from an external source.
Other methods of moving items between Kicad libraries are discussed here.
Moving items in script form can also be used to populate libraries.
Others may comment on this method.