FreeCAD open source CAD software supports export to several exchange formats, including ISO-10303 STEP product data files (.stp, .step). You can download the program for free and export your .fcstd files, which is essentially what fcstd to stp conversion is. Unfortunately, we have no information about any other 3rd party software capable of such conversion at this time.
Open FCSTD file Open STP fileAn open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler