Ok. Thank’you. I did it. It works fine!
i have just another question. If I render the scene with a visible white ground (by a standard material, not wrapper), the shadows are just a little colored by the color of object. If I render the scene with a wrapper material applied to the floor, and I save the render of the object as a targa file, with alpha channel (that contain the shadows) and compose them in photoshop, I obtaine the same result for the shadows (even if in the composition the ground floor is transparent) but shadows are not colored, are just in grey gradient. Do you know why?
Thank you so much for your time!