You can add JAR or zip archives to the internal CLASSPATH from within the IDE. Choose File > Mount Filesystem. In the wizard, select the Archive node. In the next panel, navigate to the JAR file.

You can browse the hierarchy of a mounted JAR file in the Explorer just like you can browse a local directory.

If you do not want the archive file displayed in the Explorer, right-click the archive's node and choose Properties. Set the Hidden property to True.