Individual items in a list, possibility for group them? Wpf

I have a stupid problem. I implemented a drag and drop. So i drag items (" questions ") from a listview to another listview. These items are put in a list from an object (" examination "). But i also have to be able to make clusters. So the examination object contains a list of questionclusters. Normally,when i drag and drop a question, this come in a questioncluster, and the questioncluster contains a list of questio开发者_如何学Cns. Default, this list contains only 1 question. But the user has to got also the possibility to add multiple questions to 1 cluster.My problem is that i don't really know how that i practically implement this in the gui. Do i have to create a button, for when they want to add multiple questions in the questioncluste, if on click, a pop up appears. but this looks a bit strange to me, then i have to implement the drag and drop again... with the 2 lists.. Does anyone have some ideas for implement this in a user-friendly way? Maybe working with multiple colours? when they belong to the same cluster? I am implementing in wpf , C#.

Thanks!

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I may not have correctly understood the problem, but shouldn't simply grouping the items in listview (group by cluster) help here?