I recommend using a flat ground plane and then adding the Mirror shader to it. This will render as if the sky were present above and below the horizon line, but you'll have to be careful where you place the obejct in your scene, because they will cause a reflection in the ground plane. Tilting the ground plane a little will fix this sometimes, but it will also throw off your mirror of the sky. Here is an example, of what this could look like all from from Carrara 3 Render.