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?


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