No, the OEM rack for the Boxster called the RTS or Roof Transport System mounts to the windshield frame and body just behind the doors. It can be used with either the hard top or the soft top (up or down).
http://www.pelicanparts.com/techarticles/box_roof_transport/box_roof_transport_page1.htm
I wouldn't recommend mounting a rack to the hardtop as it is not designed to carry any load.
There is also a aftermarket rack that is designed to attach to the rear bumper (bumperetes). This is a hitch type mount that can also be used to pull a small trailer.
http://www.willwoodsystem.com/bicycle%20carrier.htm