If you mean a surface scout by "brain region". Just look at the Scout that contains that vertex index in its Vertices field. Let me know if you refer to something else by "brain region"
Exporting sources as NIfTI creates a volume that has the same size as the Subject MRI. Thus it cannot be imported as sources (as Brainstorm defines the volume sources on a different grid):
https://neuroimage.usc.edu/brainstorm/Tutorials/TutVolSource