How To Change Post Author Default?

You just create must-use plugin for your multisite instalation (https://codex.wordpress.org/Must_Use_Plugins), hook to post save action there (https://codex.wordpress.org/Plugin_API/Action_Reference/save_post), and inside action callback check if current user is admin/super admin (https://codex.wordpress.org/Function_Reference/is_super_admin), if so, you find current blog user with ‘author’ role (https://codex.wordpress.org/Function_Reference/get_users) and update post (https://codex.wordpress.org/Function_Reference/wp_update_post) with his ID as author ID. At least it is first, quick&dirty solution which comes to mind.