in wpf Treeview, How I can combine multiple Items as childes into one grouped Item in Lower Level of treeview not upper level?

Question

I need to affect the TreeviewItem Class or make inherited Class, so that I can Implement grouping for multi childes as ItemsPresenter into one grouped item like the following figure:

Probably, Adding another ContentPresenter in Lower-Level of TreeView Template below than Itemspresenter is one solution OR may be to add new Parent Variable to be shared between multichildes in treeview for the inherited TreeviewItem Class, What Would be the Solution for that? Any Idea Please !

I am trying to Represent more Efficient Treeview logic!