A typical stp to igs conversion represents transformation of a 3D model (part usually) between two different format. As both the IGES/IGS and STP/STEP are fairly common CAD formats used for exchange purposes, should be pretty easy to find a CAD / 3D modeling software that can convert stp to igs or even vice versa.
Open STP file Open IGS file3D viewer and converter with huge file type support
A CAD viewer program with conversion capabilities
A set of 3D modeling tools
A 3D modelling software
A converter for 3D formats
A CAD conversion software
A CAD converter with huge format support
A CAD visualizaton, analyzation and translating system
An open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler
A 3D CAD translation software
A suite of CAD applications
A dedicated CAD converter