You can use special plugins for SktechUp, that allows users to import back their STL models that were previously exported with the program. Probably the easiest choice for this conversion, although it may be possible in other 3D modeling programs with SketchUp support.
Convert Stereolithography CAD file to SketchUp graphics.
We found 3 software records in the database related to .stl to .skp file format conversion.
Microsoft Windows software - convert stl to skp on Windows
A 3D CAD translation softwate
Convert from stl file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned stl to skp. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A converter for 3D formats
Multiplatform software - convert stl to skp on any platform
Extension for SketchUp tool for importing and exporting *.stl files