FreeCAD supports export to a stereolithography format for 3D printing (*.stl) so you can *.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 your only choice for FCSTD to STL conversion.
Convert FreeCAD drawing to Stereolithography CAD file.
We found one software available in the database for this conversion.
Multiplatform software - convert fcstd to stl on any platform
An open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler
Convert from fcstd file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.