step to mb conversion seems to be mainly related to a transformation of models from ISO STEP product data files (.stp, .step) to a format used in Autodesk Maya (.ma, .mb).
It appears that Autodesk Maya offers Autodesk Translation Framework (ATF) function that allows users to import several CAD/3D modeling formats to the program, including STEP format. You can learn more about this function in latest Maya User Guide.
If you do not have access to Maya, the only viable alternative would be to use some of the dedicated CAD conversion programs that offers users tool to convert step to mb.