Autodesk Maya supports export to STEP format (.stp, .step) for many years now, which means that it should be not difficult to use for such conversion simply through the File ► Export function.
Without Maya unfortunately, it will be very difficult to achieve export of any kind, so if you do not have access to Maya, you will have to find someone that does have ti and can help you out with the export. Perhaps some online community can help you out as people often help with exports if you send them your files.