How do I convert STL model to a solid model?

It is possible for a STL ceated from a single part (not an assembly).
1st convert stl file to dwg file (with reaconverter f.ex)
2nd download tailors lisp functions in Autocad -> http://www.polyface.de/Tailors/English/ReadMe.html
Than use mesh or face to solid to convert.
Don't expect too much from this, but if you only need somthing measurable that's good.

Best regards.

get MESHLAB program ( its free ) from autodesk.