*.csg is something different, I believe.
OpenSCAD (*.scad) files *can* be imported: https://wiki.freecadweb.org/Import_OpenSCAD_code (provided you have the application installed and follow the instructions in the link).
This will not help much, however, I'm afraid. The radii still appear to be ragged somehow:
But someone with more patience than me might be able to delete and recreate them in FreeCAD.