New eagle library: reuse standard package & symbol

You can just copy a package from another library into your own library and edit it. Are you asking how to copy between libraries?

1. Open your own library (the one you want to copy to).
2. Then bring the Control Panel window to the front (leaving your library open in the background).
3. In the list to the left, expand Libraries and expand the name of the library you want to copy from.
4. Locate the package or footprint you want to copy, right click it, and select Copy to Library.
5. Bring the window with your own library to the front, locate the newly copied package and edit it.