The Blackhawks also announced Mills had signed a one-year deal. He was previously under contract with the IceHogs.
The 22-year-old Morin was among the Blackhawks' final training camp roster cuts this season. He had a team-high 34 shots on goal and was tied for a team-best four goals in eight games with the IceHogs this season. He has played in 15 career NHL games.
The 30-year-old Mills had two goals and two assists with the IceHogs this season. He's played in 31 career NHL games and last appeared in the NHL with the New Jersey Devils during the 2011-12 season.
The Blackhawks also placed defenseman Mike Kostka on injured reserve retroactive to Oct. 22. The Blackhawks announced Wednesday that Kostka would miss 4-6 weeks with a lower-body injury.
The Blackhawks currently have 23 active roster players, including 14 forwards, seven defensemen and two goaltenders.