You don't need to create custom shape, but you can just set the Controls you want in the Content property of the regular RadDiagramShape. The major thing of having custom shape is if you need to provide custom serialization or if you need a specific properties/logic associated with it. In case you want to have custom shape, than you'll need to get the original shape's template and modify it in such a way that preserves the Connectors itemscontrol inside.
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