Dear all,
I post the solution in case any one encounters the same problem,
You just need to use "Tools/Merge Mesh" in the menu, and thanks to dear Mingrui, the toolbox creates the new surface for you:
First choose ICBM152left_smoothed.nv as the left mesh, and the Cerebellum.nv as the right mesh. This creates a smooth surface for the left hemisphere that covers both the cortex and the cerebellum.
Then do the exact same for the right hemisphere.
And finally, choose these two new meshes as your left and right mesh in the Merge Mesh menu, and you will have one smoothed surface that covers both the cortex and the cerebellum.
Best
Zahra