Why not simply create a JDialog and put your interface object into its contentPane, then pack it and display it. Simple.
You state:
Obviously JOptionPane is small, but to do a JDialog I'd have to create a Window and more stuff. The GUI is already created, and I can simply import that into the JOptionPane. Can't do that for a JDialog I believe.
I'm confused. What do you mean by "the GUI"? Again, if it is a JPanel, again, simply put it into your JDialog.
For instance if in an ActionListener
public void actionPerformed(ActionEvent e) {
// assuming this is being called from within a JFrame named myFrame
JDialog dialog = new JDialog(myFrame, "Dialog Title", ModalityType.APPLICATION_MODAL);
dialog.getContentPane().add(interface);
dialog.pack();
dialog.setLocationRelativeTo(myFrame);
dialog.setVisible(true);
}
Another option is to use the JOptionPane constructor, and create it without showing it, and then convert it to a JDialog as the JOptionPane API shows you how to do, but you may still have the OK or similar button to deal with.