Hi, 

I just came across this question too, and I think I have figured it out by looking in the REX gui. If you open up REX in the seed-voxel analysis results window, you can pull the correlation (or other measure) results for each subject and ROI by importing the Harvard-Oxford atlas in the "ROIs" selection.

So for example, I change the ROIs from the clusters found (from your first level analyses) to the Harvard-Oxford atlas (from the /conn/rois path). Then I change the measure I'm interested in to say, positive-voxels count. Then when I hit "Extract", I see the highlight ROI used in the bottom right corner of the REX display. As you can see from the attached REX display, the Frontal Pole R is not just a 5mm sphere but rather the whole atlas defined region. You can do the same for any of the correlation measurements too. Hope this helps. 

Best,
Mitch