Actually there is. Due to a bug/glitch/failing in Anim8or's scanline renderer, if you set an object's material as completely transparent, it'll still cast shadows. However I don't think this will be completely useful for adding shadows into real life photos since transparent objects can't receive shadows.
So the best way I suggest is to render out the objects without shadows, then do another render with the fake ground/floors/walls etc that'll be receiving the shadows except that all the objects are given a white material so that only the shadows stand out (and the scene background made pure white), then layer that render over the original in photoshop with darken or multiply filter applied to it.