During the process of creating a new component, non included in the standard library of Eagle, we have to create the package for the new device. Usually the package is already present in some other library, we can import the package already present with a simple copy operation.
Here follows an example. If, for instance, we are going to create a component using the SO24W package, we can look for it in any other library present in the standard eagle components library.
A simple copy command during the design of the new component will import the package saving time to the programmer and avoiding mistakes on the new design.
That's all