// Setup new mofdal window class for popupManager
protected function setupOverlay(type:Class):void{
var ipm:IPopUpManager =Singleton.getInstance(“mx.managers::IPopUpManager”) as PopUpManager;
var o:Object = ipm;
var p:QName = new QName(mx_internal, “modalWindowClass”);
o[p] = type;
}

Just make sure the type class reference extends UIComponent :)

I have been Doing quite a bit with the PopUpManager, and found it way too locked down, which is why I have to step into mx_internal namespace :(