however, in order to cast 3d light in a 2d space oyu have to do whats called matte material mapping. That is where you take an object and assign it the matte material. This essentially makes it invisible. However you can add a light and suddenly whatever characteristics the object had become illuminated or shadowed, however it appears transparently over top of the object.
However this requires you to create an exact replica of what you're trying to give shadow, which is much tougher than it sounds. And this isn't even taking into account rotoscoping.
Best advice you could take. There are ways to do it 3d, as well as ways to relight a scene in your comp package. These methods are usually reserved for a fix/repair situation or very subtle changes for mood. If you try to rely on 3d/software to "light" a scene then you're putting yourself in line for a world of headaches.
Remember the golden rule for post, garbage in = garbage out!