I've imported an .stl file and it shows up simply as an "STL Graphic" in my design tree.  I would like to make some simple cuts in the part, but SW behaves as if there is no geometry there (e.g. I try and make a cut but the option to cut is unavailable).  Is there a way to make this file a solid so that I can manipulate its geoemtry?  I'm only doing very simple manipulations...a few cuts and maybe some protrusions.