I would like share that i found out a way to get geometry out of .rsdoc files.
Since DesignSpark mechanical won't let you safe anything but their native format (.rsdoc) and mesh formats the procedure is a bit cumbersome. (I needed MS Windows and DesignSpark mechanical to perform the important steps.)
1. retrieve the .rsdoc file
2. open the file as zip-archive and extract every .sab (ACIS binary) files.
3. next you need to convert the .sab to a .sat (ACIS text) file. If you got DesignSpark mechanical installed you can use the program "SabSatConverter.exe" to do that.
4. next you need to convert the .sat file to a .brep or .step file. I used the evaluation version of the CAD Exchanger by Roman Lygin (http://www.cadexchanger.com
5. open the .brep or .step files in FreeCAD
I don't want to encourage anyone in using propitiatory software.
But I get annoyed easily, when people tell me they want to modify a stl file from thingiverse, and i can tell from the screenshot, that the original author used a software that allows exporting STEP files.
Please avoid using meshes whenever you can.