This is just a very brief note, mostly for my personal reference..
Tutorial: http://northernhope.blogspot.com/2013/05/stl-to-step-with-freecad.html
Some additions to the above tutorial: Part > Convert To Solid before exporting
Upon opening in autodesk inventor the part will be solid and you can modify faces and create sketches upon them, much better than the native STL import in autodesk.