I am not quite sure I understand the result you are trying to achieve, but basically, you can customize the appearance of the timeline items using the ItemTemplateSelector or the TimelineItemTemplate and TimelineInstantItemTemplate properties.
The ItemTemplateSelector approach is demonstrated in our Custom ItemTemplate demo. The online demo is for Silverlight, but the code is the same for WPF. If you have installed the dev/trial version, you can open the demos solution in VS and review the code of the demo locally - just go to Start Menu -> Telerik -> RadControls for WPF Q2 2012 -> Open C# Demos in Visual Studio.
the Telerik team