[Freesurfer] Volume ROI statistics

Hi, I want to obtain volume statistics from a ROI in fsaverage. I have register.dat and a .nii file (the ROI), but I don't now how to get to the .label.

The last try has been: mri_vol2vol --mov $SUBJECTS_DIR/fsaverage/label/ROI.nii --o $SUBJECTS_DIR/fsaverage/label/ROI.mgh --reg $SUBJECTS_DIR/fsaverage/mri/transforms/my_own_register_created_with_bbregister.dat --targ fsaverage

(but it is failing, error: corRead(): can't open file $SUBJECTS_DIR/fsaverage/COR-.info)

My next steps were: cor2label (in fsaverage) mri_label2label (per every subject) mri_segstats (per every subject)

Is there any simpler method to accomplish the same? Or at least that it works? many thanks, Gari