The Telerik DragDrop is not based on the WPF built-in DragDrop and it does not use the same events. If the WF designer is listening for them then I am afraid I cannot suggest how the two DragDrops can be integrated.
To be honest I have not tried to integrate DragDrop and the WorkFlow designer. If dropping is achieved with custom code, then I may be able to convert it for you so that it uses the TelerikDragDrop.
I am not sure how the designer works and you may achieve better integration with the built-in DragDrop.
Please also note that the Telerik DragDrop works only for object within the same Window, i.e. it does not work for drag-drops across applications or windows.
All the best,
the Telerik team