So when a ticket is closed, it has a username of "System". You can change this by editing two files:

In: /plugins/support_manager/language/en_us/client_tickets.php
Find: $lang['ClientTickets.reply.system'] = "System";

In: /plugins/support_manager/language/en_us/admin_tickets.php

Find: $lang['AdminTickets.reply.system'] = "System";