On the contrary, 1.6ST had a mod for doing this written by Scottro.
Available at:
http://www.sudobash.net/osticket-send-reply-to-alternateadditional-email-addresses(http://www.sudobash.net/osticket-send-reply-to-alternateadditional-email-addresses)
And 1.7ST has a mod (which is an updated version of Scottro's mod but without the BCC option).
Available at:
http://www.tmib.net/mod-staff-cc-ticket-responses-osticket-17st(http://www.tmib.net/mod-staff-cc-ticket-responses-osticket-17st)
Both were cross posted in the mods section of the forums (or at least there was a link from the mods section to the external sites).