To add or edit an Approved Repairer, follow the steps outlined in: http://www.gearbox.support/knowledge-base/add-repairers/
Once you have created the repairer or are editing an existing repairer, you can check the ‘Approved Repairer’ checkbox (1) and click Update Repairer (2):
Once you have updated the repairer, you will be taken back to the Repairers screen – you can click on the Approved Repairers button (1) and it will show you all repairers marked as approved: