There's no support for custom elements in the designer at this time. If you need to provide users with templates as a starting point only, you could add some ShapeNodes to a NodeListView and import their shape upon drag-and-drop:

<diag:ShapeDesigner
	x:Name="designer"
	Height="200" Width="300"
	AllowDrop="True"
	DragOver="OnDesignerDrag"
	Drop="OnDesignerDrop" />

private void OnDesignerDrag(object sender, DragEventArgs e)
{
	e.Effects = DragDropEffects.Copy;
	e.Handled = true;
}

private void OnDesignerDrop(object sender, DragEventArgs e)
{
	var draggedNode = (DraggedNode)e.Data.GetData(typeof(DraggedNode));
	var shapeNode = (ShapeNode)draggedNode.Node;
	designer.ImportShape(shapeNode.Shape);
	e.Effects = DragDropEffects.Copy;
	e.Handled = true;
} 

We are improving the designer for upcoming release (e.g. to show grid and allow defining anchor points), and could also implement a method for adding new templates if it will help you. The templates in the designer's palette are lists of ElementTemplate -derived objects, so you will be able to add only predefined Shape.Outline lists there instead of full shape's definitions (including decoration lines and filled decorations).

The Primitives palette shows single ElementTemplate -derived objects, and if you need to add to it you'd have to implement both a new ElementTemplate class and a companion builder class used by the designer to represent the element.

I hope that helps,
Stoyan