Hello Ola Karlsson,
There is a blogpost dealing with this question here. You cannot add or remove buttons to the default SelectionMiniToolBar, but you can create your custom selection mini toolbar, implementing ISelectionMiniToolBar and mark it with the [CustomSelectionMiniToolBar] attribute. It will then be automatically loaded by MEF. if you do not want to use MEF, you can manually invoke:
Try it out and let us know if you encounter any difficulties.
the Telerik team
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