Mack signed with the Nittany Lions out of high school and played in all 13 games in his first season before transferring to Alabama.
Buffalo will be Roberson's fourth program in his seventh year of college football.
Walfe, who has four years of eligibility remaining, transferred from Michigan.
The junior announced on Saturday he would return for his senior year.
No, Drew Allar's interception wasn't the only reason behind Penn State's elimination from the College Football Playoff.
The Capital One Bank Guy is actually a pretty good juggler.