Cure cancer.
Seriously though - that's a very difficult thing to do, does anybody know whether Geomagic can do this? Geomagic is software specifically for data translations/creation from and to scanning devices and so on. So it should be quite comfortable with .STL files.
Even if it works, your results may vary in terms of quality, what kind of geometry is in the .STL file? And for what purposes will it eventually serve as an .IGS?