If you want to put a 2-d image for the sky, you could create a GraphicsLayer that would be drawn first (give it a low sort index), and set up an orthographic camera and its own scene graph. It would be just like render2d–see ShowBase.makeCamera2d()–except it would be drawn first (behind everything) instead of last (on top of everything). Then you could put whatever 2-d stuff you like in this scene graph, and it would be drawn behind everything else in the regular 3-d scene graph.
Another, simpler thing to do this would be simply to parent your sky geometry to the camera, far enough away that it would be behind everything in your scene.