What you can do, then, is create a generic interface...something like:
public interface MyInterface {
public void execute();
}
Have the ActionListener that responds to the default button press event also implement this interface. Then pass this listener object (as type MyInterface) to your KeyListener and call the execute() method when 'ENTER' is pressed. In this way, you can always respond correctly to the 'ENTER' key press and you don't care what the default button is. If I am not mistaken, this is something similar to the Command design
pattern.
Hope this helps.