Using older versions of Outlook for Mac in business environments
Did you try: Click on 'New email'. Look for the little square on the top right corner of the email box. When you find it, it will indicate 'Open in new window'. Click on it. Your new window will have a weird blue line around the 'to' field, but it should work.
Likewise, if you are replying to someone and want to add another address, go to 'open in new window' and you should be OK.
This works for me - let us know if it works for you.
Ann