You can most likely find utilities that can help you directly convert models saved in Autodesk Kaydara 3D format (.fbx) to ISO STEP product data files (.stp, .step), but the support for this fbx to step conversion seems to be pretty limited.
Alternative would be to use utility called FBX Converter, with which users can easily convert Kaydara 3D data (.fbx) to some common 3D modeling formats like .obj, which can then be converted to .stp using any of the available obj to stp converters.