PS:
I've just added a missing link between the volume parcellations and the source analysis.
Now they can be imported directly as volume scouts.
Documentation: https://neuroimage.usc.edu/brainstorm/Tutorials/TutVolSource#Volume_atlases
Commit: https://github.com/brainstorm-tools/brainstorm3/commit/2ca2cd7bcd24fa4c9588e460b84991c8d57e183b