There is actually a way to do this in FreeCAD. Tried it with a simple model to see if this works,
see screenshots.

After opening the converted model in Moi, it shows up as a solid and is editable.
Here is a link to a youtube video that explains the process. Not sure which results you
get with more complex geometry.

FreeCAD Useful Tips and Awesome Tricks with @WayofWood! |JOKO ENGINEERING
https://youtu.be/0zxjOB2Cao8

It's the second tip. Hope this works for you. I don't think you can avoid the meshlook
on rounded/curved surfaces.

What I sometimes do:

- Convert .stl to .obj using Blender
- Use the "Obj23dmWireframe converter" from the Resources page
- Import the resulting 3dm file
- Adjust the scale using a known reference length along the object
- Lock the object
- Recreate the object by snapping to the points of the imported mesh