Hey secti,
Thanks for the clarification! You are right, that function is not available in Outlook 2011 for Mac (Honestly, I haven't seen this function in Outlook 2010 either. If you have instructions for 2010, let me know and that may help).
Outlook 2011 does support send as and send on behalf of, with the correct permissions. Once you have the permissions, and the users mailbox/calendar setup, you can change the email you are SENDING from in the "From:" Field.
A workaround for this is to CC the email address you want a reply to, and then ask the recipient to 'Reply All'.
Hope this helps! Let me know.
-Shane