The Chiefs have also signed former Crusaders centre Robbie Fruean, who recently had heart surgery.
A Chiefs' statement said Fruean, 25, had returned to full exercise training and was on track to return to contact training and playing in early 2014.
"I am thankful for the chance to return to rugby at this level and am really looking forward to the Chiefs environment and the beginning of a new and important chapter for me," Fruean said.
Chiefs coach Dave Rennie said: "Mils and Robbie are both proven talents and top men. We know the value they will add to our side.
"Robbie is big and powerful and Mils is one of the most experienced players in New Zealand rugby so both are great additions to our squad."