There is a method called “Shadow Catching” which basically sets up a material surface to only catch shadows. By doing that you can e.g. have a photo as background in Daz Studio and catch shadows on surfaces of a 3D object which is shaped similar to the object in the actual photo.
I am not sure if there is any recent tutorial for this. Carnite did a video tutorial on youtube for Daz Studio 3. Since the Shader Mixer (which is what the tutorial uses) changed quite a bit from Daz Studio 3 to 4.5, the whole Shader Mixer layout and functions changed quite a bit, so the tutorial likely cannot be applied 1:1.
Part 1: http://www.youtube.com/watch?v=qo2OVy9tf8E
Part 2: http://www.youtube.com/watch?v=rrWtXnijQ_k
Additionally there is there is the pwCatch product in Daz store, which is supposed to do something similar, it is basically a shader for surfaces to dabble with shadows. I don’t own the pwCatch product, so don’t know how this is applied and what features it got.
Edit: ah hehe, Porsimo was a bit quicker, shouldn’t write that much