Revit :: Custom Window Family - Cut Geometry Not Allowed

I am semi-new to Revit and am trying to make a go of it.  I am trying to make a window, that looks like my AutoCAD window.  So I open the Window Family.rft to start creating.  I can adjust my wall thickness to match my brick wall, I can set the frame to the void that is already there.....my problem comes when I want to put on the brick mould.  This overhangs the window frame and thus is outside the pre-ordained void.  When I try to make a void extrusion to cut back the outside face of the wall and allow the brick mould to show, it will not work - only the originally supplied void (that comes with the template) works.  

Revit will not allow me to "Cut Geometry" saying "A Family cannot have an opening and cut in the same host". If I set the brick mould inside the supplied opening, then the inside face will have a gap between the drywall and the window frame.