Sometimes you need to use a device – found in a project downloaded from Internet – in one of your projects; here’s how to do it in Eagle:
First, if you don’t already have a personal library, let’s create an empty one.
From Eagle’s main window, open File menu and choose New > Library:

The library editor will show, for now simply save the library just created: File, Save as…

Choose a name for your library and click Save:

From File menu, click Run…; then choose exp-project-lbr.ulp and click Open:

Let the default options and click Collect data: the script will analyze the file and list – in the textarea – all the devices it will found (this step could last several seconds).

When the script ends, click Create library to start the real export process:

Sometimes Eagle asks wether to export also only mechanical packages, click No:

At the end of the process, in the same folder you saved the .sch file, you’ll find the new libraries created by script.

If you click on the triangle on the left of the name, you can show library’s content:

Now open your personal library and, with drag’n’drop, drag inside it the device you need, grabbing it from one of the new libraries:

The device (symbol + package) will be added to your library.

For more details,please refer to original post

http://www.lucadentella.it/en/2011/11/04/eagle-come-esportare-un-componente-da-un-progetto/