Adding geometry to .stl file

Hello, is it possible to add CSG geometry (such as a box or plane for boundary) to .stl file? I imported the .stl file via netgen.stl and being able to mesh it and save as .vol file but to perform Navier Stokes simulation via the geometry, I need to add box like geometry for boundary. You can have a look at the .stl file.

Hi,
not directly, for such a case probably the most easy thing is to mesh the box and the object seperately and merge the surface meshes. Then you can create a volume mesh from the merged mesh, here is a tutorial on how to do this:
ngsolve.org/docu/latest/netgen_tutorials...ing_with_meshes.html
Best