[Freesurfer] MNI to RAS surface coordinates

Dear Freesurfer experts,

I have performed a VBM analysis with SPM, using some VOIs. Now I would like to compare the volumetric results obtained with SPM with those of cortical thickness obtained with Freesurfer in the same VOIs (approximately).

For this purpose, I would like to transform the MNI coordinates of my SPM VOIs into RAS surface coordinates of fsaverage, to draw a label of more or less the same size of the SPM VOI and to propage it to all the subjects in order to obtain an average cortical thickness of this label (VOI) for each subject.

1) Would this procedure be correct?

2) How could I transform my MNI coordinates into RAS surface coordinates?

Thank you in advance, Yolanda