Hi Roberto, Ed and Richard get paid to post answers really quickly. Unfortunately, they get paid whether or not the answer has anything to do with the question that was asked. Unpaid volunteers like me get stuck cleaning up after them. The answer to your question is that the Quick Access toolbar can not be customized in Outlook for Mac.
Microsoft uses a separate forum to accept and reply to feature requests. Your request already exists at this link, where you can vote for and comment on it. I am an unpaid volunteer and do not work for Microsoft. 'Independent Advisors' work for contractors hired by Microsoft.
'Microsoft Agents' work for Microsoft Support.
Outlook for Mac 2016 (version 16.16.4 (181110)) Mac OS version 10.13.4 I was given this feedback when I submitted my Outlook web add-in to the store for testing.