This is in NetBeans 6.5, Java 6.
I have the following hierarchy in the NetBeans GUI Designer:
JFrame
JTabbedPane
JPanel X
<...>
Create a separate JPanel class. Customize JPanel as needed. You can then drag-and-drop the JPanel class onto the Form Designer. This is exactly what I do.
Alternatively, you can click on Use Bean and then type in the name of the class.
Alternatively, you can do an Add from the pallete and it will scan your JAR for any "beans". It should pick up your custom JPanel as well.