Can be installed via the AddonManager in Tools menu -> AddonManager if you have a very recent build. In the AddonManger click Configure, then add https://github.com/mwganson/MeshRemodel
to the list of custom repositories. After restarting the AddonManager you should find MeshRemodel in the list of workbenches you can install.
Full documentation is at that url.
This workbench is for helping in remodeling a mesh object into a native FreeCAD object. The general workflow is to select the mesh object in the tree, then click the create points tool to create a points object, then use these points as references for creating elements, such as lines, polygons, circles, arcs, and bsplines. These elements can be created directly in the 3d view using the points as references. For example, select 2 points, then click the create line tool to make a line between those 2 points. Select 3 points, then select the create circle tool to create a circle or an arc. Once you have enough elements created to form a continuous wire, you can select the elements then use the create wire tool (which uses Draft.upgrade) to connect them into a single wire object. Once you have all the wires created for a sketch, select them in the 3d view, and click the Create sketch tool to make a sketch (uses Draft.makeSketch()). For convenience there is also a merge sketches icon, which uses the Sketcher merge sketches tool.
There is a tool to filter the points into only those that lie on a single plane. To do this select 3 points, the click the create coplanar points object tool to create a new points object containing only those points that are coplanar with the 3 selected points. This object is a compound of Part Points, which is automatically exploded. Press Undo (Ctrl+Z) if you would prefer it to not be exploded. The advantage of the exploded form is you can use block selection (Shift+B, draw rectangle) to select multiple points simultaneously. For example, use block select to select points in a circle, then click the make circle tool to make the circle. Tools that support block selection at this time: bspline, circle, polygon.
The polygon is actually a compound made up of lines. This is done as a separate document transaction, meaning if you click Undo (Ctrl+Z) after making a polygon you will get the individual lines as separate objects. This can be useful where you would like to delete some of the lines individually.
This workbench is still under construction, but I think it's at the point where it's usable. Here is a video demonstrating the remodeling of a very simple imported mesh.
The element creation tools are compatible with any selected points. For example, you could select points from a sketch (not in edit mode), from a cube, etc. and use those selected points as references for making lines, polygons, circles, arcs, and bsplines. So, it can potentially be of use even outside the context of remodeling a mesh.
If you prefer to model inside the sketcher you could use the points objects as links to external geometry. One advantage of working outside the sketcher as I did in the video is I can now delete all of the points objects and elements because there are no dependencies like you would have using links to external geometry, as seen in this screenshot of the dependency graph:
The only dependencies the extrude objects have is to the sketches, which have no outside dependencies.
Using keyboard shortcuts, the work can go quite fast for these circles. Shift+B to go into block selection mode, draw rectangle to get at least 3 of the points selected, then Alt+R,Alt+C to make the circle.
Here is another video demonstrating how to connect arcs and lines into a profile.
The polygon (inner square) had some missing coincident contraints in the sketch. This wasn't an issue when I was making the polygons as single entities, but I switch to making them out of individual lines, then compounding them into a single compound. The reason for the switch is to be able to press Undo (Ctrl+Z) to get at the individual lines for deleting them individually. Edit: polygons missing coincidences is now solved in version 1.21.
It is also possible to create the outer profile as a bspline by selecting all the points, but I would prefer to have arcs in this case for easier editing in the sketcher. The bsplines can be useful where you have an ellipse in the mesh. I haven't worked out the math for getting an ellipse out of 5 selected points yet, and probably won't bother continuing since the bspline seems to work quite well. In such a use case you would want to use block select mode because there are likely to be quite a few points to select, which would be tedious to do one at the time. The problem with block select is the ordering of the points becomes crisscrossed, so if you just make the bspline (or polygon) you get crossed lines. Therefore I have a sorting algorithm built in to sort the lines. Alt+Click to use this sorting on bsplines and polygons.