The 36-year-old Looper, who had a non-guaranteed minor league deal, posted a 9.95 ERA in 12 1/3 innings over five appearances this spring. He missed the entire 2010 season after struggling with injuries in 2009 with the Milwaukee Brewers. Looper finished with a 72-65 record and a 4.15 ERA in 12 major league seasons.
"He certainly had an outstanding career," Cubs general manager Jim Hendry said. "[Having him in camp] really was a good, sound idea and I'm glad we did it."
The move helps Carlos Silva's bid for a spot on the Opening Day roster as either a starter or reliever. Silva has been competing with Looper and Andrew Cashner for the fifth spot in the rotation, and Cubs manager Mike Quade said he planned to make a decision by Saturday.
Samardzija and Russell join Carlos Marmol, Sean Marshall, Kerry Wood and John Grabow in the bullpen with one spot left to fill. Quade said will Cashner be on the 25-man roster either as a starter or reliever. He is scheduled to pitch on Saturday against the Texas Rangers.
Russell joins Marshall and Grabow to give the Cubs three left-handers in the bullpen.
Silva, who has an $11.5 million guaranteed contract, has a 10.90 ERA in 17 1/3 innings this spring.
"I wanted to tell Wellsy because I'm sure he's tired of talking to you guys [the media] about 'Are you or aren't you the fourth starter?'" Quade said.
A major league source confirmed that Koyie Hill will be the Cubs backup catcher to start the season.
Bruce Levine and Jon Greenberg contributed to this report.