I think I know what Shondoit is asking for. He basically wants to give Window class custom properties.
Like:
code:
function createWindow(){
var WndTest = MsgPlus.CreateWnd("interface.xml", "WndTest");
WndTest.CustomProperty = "This is a custom property.";
}
function OnWndTestEvent_CtrlClicked(PlusWnd, ControlId){
Debug.Trace(PlusWnd.CustomProperty);
}