c# – HelixToolkit scene manipulation

I’m using HelixToolkit to import a model and display it.
Now I want to take one of the objects of the model and rotate it.
Unfortunately I can’t find a way to edit the scene the importer gives me.

var imp = new HelixToolkit.SharpDX.Core.Assimp.Importer();

var scene = imp.Load(".\test.obj");

foreach (var node in scene.Root.Traverse().ToList())
    if (node.Name.Contains("gate"))
        node.RemoveSelf(); // remove from scene to be able to add to group
        var mg = new SceneNodeGroupModel3D();
        mg.Transform = new RotateTransform3D(new AxisAngleRotation3D(new Vector3D(1,0,0), 45 ));
        // could not find a way to add the group back

this.ModelGroup.AddNode(scene.Root); // That's the SceneNodeGroupModel3D that is bound to the Viewport3DX for displaying

While I can remove the object and add it to the rotation group I can’t add it back to the scene.

The SceneNodeGroupModel3Ds Parent property is not settable and a SceneNode also has no way to add children to it.

So How does scene editing work with HelixToolkit”?