Kackert played three seasons with Toronto, rushing for 1,467 yards and 12 touchdowns on 228 carries for a 6.4-yard average. He added 50 catches for 454 yards and two touchdowns.
The running back was the 2012 Grey Cup MVP after rushing for 133 yards on 20 carries in the Argos' 35-22 win over the Calgary Stampeders at Rogers Centre.
He broke his left ankle prior to the 2013 East Division final and retired on the first day of training camp in 2014 because the injury hadn't healed sufficiently.
He attempted a comeback later in 2014 but was released after suffering a right hamstring injury and joined the team as a strength and conditioning coach.
Stala, a 2010 CFL all-star receiver, has 418 receptions for 5,314 yards and 27 touchdowns over 12 seasons with Montreal and Hamilton.
Whitaker spent six seasons as a running back with Montreal, rushing for 3,376 yards 13 touchdowns and adding 1,659 receiving yards and 13 touchdowns.
He was a first-team CFL and East Division all-star in 2011 after leading the CFL in rushing. He won Grey Cup Championships in 2009 and 2010.
The Argos also signed offensive lineman Scott Mitchell to a practice roster agreement and released running back Lyle McCombs.