Adding multiple Case Coordinators in a Drupal webform?
Technically it's the permission "access all cases and activities" that can add roles on Manage Case but I guess that's the same as "site admin"/"full access" in your setup. The reason seems to be to avoid a loophole where access to cases could be granted that shouldn't be (see https://civicrm.stackexchange.com/a/33409/181).

Anyway what you might be able to do is tag or group the people who should all be case managers for a given local site, and then using hook_postProcess or hook_post add some code that automatically adds those roles when a new case is created for people who have the same tag/group as the case creator, i.e.

loop for each person with the same tag/group as the logged in user:
civicrm_api3('Relationship', 'create', [
  'contact_id_a' =&gt; 123, // client id
  'contact_id_b' =&gt; 194, // other person's id
  'relationship_type_id' =&gt; 9, // Case Coordinator is
  'case_id' =&gt; 12345,
]);