public static DiagramLayer AddDiagramLayer( 
   Map map,
   NetworkDiagram networkDiagram
)

{
  // Create a diagram layer from a NetworkDiagram
  static void CreateDiagramLayerFromNetworkDiagram(NetworkDiagram networkDiagram)
  {
    // Create the diagram map
    Map newMap = MapFactory.Instance.CreateMap(networkDiagram.Name, MapType.NetworkDiagram, MapViewingMode.Map);

    // Open the diagram map
    Task<IMapPane> mapPane = ProApp.Panes.CreateMapPaneAsync(newMap, MapViewingMode.Map);

    //Add the diagram to the map
    DiagramLayer diagramLayer = newMap.AddDiagramLayer(networkDiagram);
  }
}