FreeCAD supports export to a stereolithography format for 3D printing (.stl), so you should be able to convert .fcstd files to .stl using this software. As far as we know, there is no other program that supports FreeCAD formats, so the program is likely your only choice for fcstd to stl conversion.
Open FCSTD file Open STL fileAn open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler