Mitchell signed a $5.4 million extension, and Reto inked a $4.35 million deal, the Avalanche said Friday.
Mitchell, 29, signed a free-agent deal with Colorado in July 2012. He has seven goals and 16 assists in 65 games this season.
Colorado acquired the 27-year-old Berra from Calgary on March 5 for a second-round draft pick. Berra, who represented Sweden in the Sochi Olympics, was 9-17-2 in 29 games for the Flames.
Berra is scheduled to make his Avalanche debut on Wednesday at Winnipeg.