It is possible.  What you need to do is hide the Ground prop and use a square ground plate as your surface.  The plate can be found in the Props Primitive folder.  Select the ground plate and enter the material room, open the Wacro panel on the right and click Set up Shadow Catcher (Yellow Highlight).  Clicking the button will activate the Shadow_Catch_Only (Red Highlight) check box on the PoserSurface.  Set both Diffuse_Color and Specular_Color to black (Green Highlight),  

                                                                            

Make sure that the figure/object is positioned on the ground plate and the ground plate placed in its final position.  I recommend that the Cast shadows and Light emitter features for the ground plate be turned off.  Position your light accordingly and tweak the settings so the shadows match the background image.  Assuming that you are using the Firefly render engine, you should have an image that is posted below: