Both the STL and IGS CAD files are pretty common, so you should have no trouble in finding a program that is capable of convert STL CAD files to IGES/IGS.
Convert Stereolithography CAD file to IGES 3D exchange format.
We found 19 software records in the database related to .stl to .igs file format conversion.
Microsoft Windows software - convert stl to igs on Windows
A 2D and 3D CAD construction software
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 igs. 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 PLM/CAD/CAM/CAE software suite
3D viewer and converter with huge file type support
A CAD and Gerber conversion software
A family of 2D/3D CAD software products for Windows
A widely-used 3D animation application from Autodesk
A set of 3D modeling tools
A converter for 3D formats
Multiplatform software - convert stl to igs on any platform
An open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler