What I did once was rendered an animation in preview mode with the ground shadow display on. Then I rendered the animation ( not changing a thing) with the real renderer and composited it over the first one. Only problem is that the shadow will always appear as if the light is dirrectly above the character. Pict, tiff, and photoshop files render with alpha channels- background should be 100% black. You could also use the render with the shadow as background in poser. You could do the whole thing with poser.