EAGLE CAD - How To Copy A Package From A Library To Another

For start open the library in which you wish to have the package (File > Open > Library).

EAGLE CAD - Open The Target Library

Go to the Control Panel window and expand the Libraries folder (left side of the window). Locate the library that contains the package you need and expand it. By experience I will say that the libraries
ref-packages.lbr and ref-packages-longpad.lbr will do 90% of the work. You can find here almost every package you need. Find the package you need and right-click it. On the pop-up menu select Copy to library. This will copy the selected package to the opened library.

EAGLE CAD - Copy The Desired Package To The Target Library

Now the package is available in the desired library.