The OpenSCAD Module is not yet included into FreeCAD and in an early development stage.
The OpenSCAD Module offers interoperability to the open source software OpenSCAD. It conatins a Importer wichs allows to open the .csg output of OpenSCAD in FreeCAD.
The exporter outputs a CSG based (sub-)tree to .csg file. Geometry witch does is not based on CSG operations will be exported as a mesh. Therefore conversion from CSG to BREP should be possible in theory. Whereas a conversion from BREP to CSG is, in gernal, not possible.
The OpenSCAD Module contians a toolbox with functions to modify the feature tree and repair models.