Yes.
Create a Jpanel, (directly from JButton may work) and implement javax.swing.MenuElement. If the button (in the JMenu) is not itself a menu you only have to over-ride MenuElement methods as so
public Component getComponent(){return this;}
public MenuElement[] getSubElements(){return new MenuElement[0];}
public void menuSelectionChanged(boolean isIncluded){}
public void processKeyEvent(KeyEvent event, MenuElement[] path, MenuSelectionManager manager){}
public void processMouseEvent(MouseEvent event, MenuElement[] path, MenuSelectionManager manager){}
Good luck. Bamkin
Sorry, read the question wrong...
mouseEvent and keyEvent, (this could be done with a actonListener on the button) must alert a JMeuItems that they are to become visible, while they are visible menuStateChanged must return true. AND getSubElements must return an array of JMenuItems.
Bamkin