Could you please elaborate more on your particular requirement since this can be implemented in various ways? I prepared a sample for you that can be used with dlls from Q2 2012 SP (0725) or sooner. Basically, you can press Layout Button, then select an item that has a parent and press the button Add Parent Shape. This will create a corresponding item in the ViewModel connected both to the selected item and its parent. Then you can again press Layout button. Hope this will help you proceed further.
Yes this is expect. Currently the Hierarchical Layout types in the RadDiagram arrange the different layers (levels) of Shapes to be with equal X or Y coordinates. In other words, there is no out-of-the-box way to exclude some item from the current tree layouts automatically. Once the shapes are arranged (layout is finished), you will have to drag this sibling ("collaborator") item to the place you need.