<p>How do you add a JAR file to an already existing Java library in Eclipse?  Note that this is not a user library.</p>

<p>That is, if you look at the Java Build Path for a Java project and click on the Libraries tab, you will see the list of libraries used by the project.  If you expand a given library, you will see a list of JAR files included in that library.  I want to add an additional JAR file to one of these libraries.</p>

<p>I am using Version 3.4.0 of Eclipse.</p>

In Eclipse Ganymede (3.4.0):
Select the library and click "Edit" (left side of the window)
Click "User Libraries"
Select the library again and click "Add JARs"