If you have Moi just load the Step file and save it in SKP for use it inside DesignSparks ! ;)

If not you can use for example Free CAD for convert them in OBJ
http://www.freecadweb.org/wiki/index.php?title=Feature_list

then Load this OBJ inside Sketchup and save it as SKP ;)
http://sketchucation.com/forums/viewtopic.php?t=20584 (plugin obj import by TIG)