Can not find what you are looking for, the Installation and configuration manual does not shed a light on it either:
Parameter used to set which usernames require administrative privileges over all the users tables. For example, this could be used to post events in the users calendar without requiring the user to configure his/her ACLs. In this case you will need to specify those superuser’s usernames like this: SOGoSuperUsernames = ([, , …]);
Kind of strange: if you login in as the user you want to impersonate and you delegate the mail box to the admin user (or any other user); this name shows up in the drop-down menu.
first logon as “impersonate_user” and deligate:
then you can “send as”:
Note: only tested this with internal mail.