Yes, you should be able to add timeline control in the diagram if you wrap it in a RadDiagramShape
and place it above the swimlane. Then you can bind the timline's PeriodStart and end PeriodEnd to a properties from your view model. To resize the shape with the timeline accordingly to the swimlane you can bind their Width properties, this will ensure that when you resize one of the shapes the other will be resized too.
As for the phases mentioned in the second paragraph of your reply, I am not sure that I understand what you mean by phases. What are they and how exactly they should look. Can you please give me a little more description about your scenario? This will help me in better understanding it.
You can drag and drop items between any control and the diagram, but this can be achieved with custom drag/drop/serialization/deserialization logic. You can take a look at the Drag Items from a Custom Databound DiagramToolbox
help article that demonstrates drag/drop from a control populated with shapes to a RadDiagram. I also attached a project demonstrating drag/drop between ListBox and RadDiagram.
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