In the DiagramToolbox tutorial
you can find detailed information about the control and how you can customize it to better fit your needs. Please take a closer look at the Customize the RadDiagramToolbox ItemsSource Collection
section that demonstrates how to customize the default Items collection of the control to better fit your needs.
I hope this information will help you get started with your task. If you have any further questions, please let us know.
All the best,
the Telerik team