If you are just trying to hide admin account you can just set some random account as founder, login with the new account and you can remove the old account as founder, admin etc. You do not have to add the new account to the admin group, as founder by default it has full access to the ACP. The only way you would be able to determine it's founder account is with access to the DB.
As another option you can create a new group and set it as hidden, copy the permissions from the admin group.