Create JUnit Test for a .JAR file

I need to create Junit test for a .JAR file in Netbeans. I don't have access to the code in this file. Now if I use another package that I have wrote I can just right click tools-> Create Tests. If I try to do that with the Jar file, it says " No testable classes found." I tried it with the JAR file in the src folder as well as the libraries folder.