Only location owners can add managers. While you can add single managers in bulk, if you have different managers you want to add by location, it must be done manually (or via API). If you have a manager that you want to add to all or a set of your locations, you simply need to move these locations to a subfolder in GMB. This doesn’t impact any location data or verification. Once your locations are in a subfolder, edit the subfolder and ‘manage users’, adding the GMB email address of the desired manager.