I have a couple of questions:
Are you talking about arcgis viewer for flex?
If this is the viewer, is the layer list the drop down layers that you can check on and off to make layers visible?
If this is the viewer, it seems that you want to group the widget into a drop down menu. Add a <widgetgroup> tag to your config.xml and place the widgets you want to place into that tag (*make sure to close tag).
Hope this helps