Federkeil, a 29-year-old native of Medicine Hat, Alta., played college football for the University of Calgary before being signed as an undrafted free agent by the NFL's Indianapolis Colts in 2006.
He spent four seasons with the Colts, earning a Super Bowl ring in 2006, before retiring in 2010 partly due to complications from a concussion.
Federkeil was Toronto's first choice, fifth overall, in the 2006 CFL Canadian draft.
"We're very happy to welcome Dan to the Stampeders," head coach and general manager John Hufnagel said in a statement. "When we agreed to the trade on Friday, we understood there was an element of risk because Dan had been out of football for three years and we weren't sure if he would be interested in playing again.
"But when we spoke this weekend, he seemed eager to play and we were able to come to a cap-friendly agreement that rewards Dan if he makes the team and contributes."
After retiring from the NHL, Federkeil returned to Calgary to finish his degree in economics. He has been medically cleared to return to the field.
"Being settled in Calgary with my young family, playing for the Stampeders is the best scenario I could ask for to resume my career," Federkeil said. "I'm excited for the chance to play football in Calgary again."
Antwi has appeared in 32 regular season games for the Stampeders, playing primarily on special teams with some action on defence.
In his two years with the club, the Toronto native has totalled six defensive tackles and 28 special teams tackles.
"Akwasi is an athletic Canadian linebacker who can play both special teams and defence. We're excited to bring him home to Toronto," Argos GM Jim Barker said in a statement.