There's no straitforward way to add a control between the MaximizedItem and the MinimizedItems, but you ca try to achieve this with some grid columns and a couple of events.
I've created a sample project that uses a RadExpander to hide and show the minimized items so could you please examine it and see if this is what you're looking for. You can also read this article
and see how to add a GridSplitter between the MinimizedItems and the MaximizedItem.
If you have more questions please feel free to ask.
the Telerik team
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