Yes it's possible to do this but this way you won't be able to go back to a specific level e.g. currently if your path is Item 1 -> Item 1.1 -> Item 1.1.1
and etc. and you click on the header of Item 1.1 the other items will be closed and the path will be Item 1 -> Item 1.1
To implement your desired behavior you could edit the default BarItemControlTemplate and replace the split button with a dropDownButton. I've attached a sample project demonstrating this so could you please examine it and if you have more questions feel free to ask.
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