I understand what you are trying to do, but my take on this is that it is a bad idea. The concept of QlikView is to make selections and make results based on your selections. Introducing a non-existing value that does not associate properly with your data will not give the end-user a good interface.
My suggestion is that you look into using the sheet object's Special Icons instead. For example you can add a Clear icon in a list box. See Object Properties > Caption for available icons.