Im not really sure why you would want to do this, but what you could do is add the JFrame's contentPane to a JPanel, and then put it in the other one.
So lets say you have two JFrame objects: frame1 and frame2
JPanel frame2Content = new JPanel();
frame2Content.add(frame2.getContentPane());
frame1.getContentPane().add(frame2Content);
Depending on your layout manager, that would allow you to put frame2's components, as a panel, somewhere in frame1.