Using NASA CGNS Meshes, group of nodes to group of faces

I am trying to use meshes from NASA validations test cases to create Code_Saturne studies. When I import the coarsest backward facing step mesh, it detects the good groups of volumes but for the faces it creates group of nodes. I was trying to use the group filter to create group of faces with my group of nodes but I could not get things working.
Is there a trick ?
Thank you very much