Move observer to reviewer maintaining approval

When a review completes, all participants are marked as 'Approved'.  We'd like a method by which to keep track of who actually approved the review.  One way to do this is to make sure anyone that marks the review as approved is a reviewer.  However, when we move an observer to a reviewer, the review status (i.e. approved) gets cleared.

Is there a way to move the participant to the reviewer state after approval and not lose the approval status?