Hi Bek,
If my memory serves me correctly, this is possible only if you have a Admin check the review requests generated prior to being sent out for the reviews (i.e. abit of manual intervention).
If you have enabled param 2007 (Admin review required before sending tasks to reviewers) to "YES" in the SPRO maintain configuration settings, you/admin should be able to manually change the reviewer for the selected generated UAR request data under Compliance Certification Reviews > Request Review
(See below for the screen that appears).
There you should see the "Change Reviewer" button. I am sure, that where a valid Manager exists, you can keep those requests as is, but where you may have a blank Manager assignment for a user, or the Exec, you can search for these using the provided search options in the page and change the reviewer by selecting that Request.
I hope that helps.