Akmal will be available for the Sydney side for first three matches, the team said in a report on their official website. Sixers won the Champions League Twenty20 last month in South Africa.
Sixers website Akmal as saying: "I jumped at the opportunity to be a part of the Big Bash League when the Sixers approached me. I'm looking forward to spending December in Australia and will be aiming to assist them to go back to back and win the tournament again".
Sixers will play Sydney Thunder in the season opener on December 8 at Sydney Cricket Ground (SCG).
Sydney Sixers BBL 2 squad: Umar Akmal, Brad Haddin, Josh Hazlewood, Moises Henriques, Daniel Hughes, Josh Lalor, Brett Lee, Nic Maddinson, Ian Moran, Sunil Narine, Stephen O'Keefe, Kurtis Patterson, Steve Smith, Daniel Smith, Mitchell Starc, Dominic Thornely, David Warner.