Both the 3D object (.obj) and ISO STEP (.stp, .step) engineering formats are quite common and well supported in various 3D modeling / CAD software. So you should have no problem with finding some tools capable of conversion of models from .obj files to .stp files. Some good picks are listed below.
Convert 3D object graphics to ISO-10303 STEP product data.
We have found 12 software records in our database eligible for .obj to .stp file format conversion.
Microsoft Windows software - convert obj to stp on Windows
A 2D and 3D CAD construction software
Convert from obj 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 obj to stp. 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.
3D viewer and converter with huge file type support
A 3D model converter
A widely-used 3D animation application from Autodesk
A converter for 3D formats
A dedicated CAD converter
A CAD converter with huge format support
A set of 3D modeling tools
Multiplatform software - convert obj to stp on any platform
3D CAD/CAM tool for product development
An open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler