Creating a 3D modelled "mold over" an imported object
Instead of using FreeCAD, I would import the mesh data into a vertex-based 3D modeling software, such as blender.
After stitching the model in any areas where you still have gaps, you remove any vertex belonging to areas you do not want to use for the mask. This leaves you with pretty much a skin-tight base for your mask. Select all the remaining surfaces and scale S them in all directions in regards to the origin so you create a little airgap 5% extra (1.05) should create enough space to sit somewhat comfortably, but you might prefer a little more. As a stand-in for the face, I use this approximation of a forehead...

Next, we thicken that surface using the modifier menu: Select the Solidify modifier and set a thickness that looks good to you and that might be printable. Then make sure it pushes away from where your face is. Also make sure the Offset is at 1 or -1 to ensure that it does not stick out from the face-scan surface to the back!

Now you got a perfect base to model on the outside of using the sculpt features while retaining the inner surface! Do yourself a favor though: print at least part of this model as a test fit and adjust the mask as needed.