The 27-year-old Herndon was claimed by Toronto from Philadelphia on Oct. 23. The Blue Jays then designated him for assignment Oct. 31. Herndon made five appearances for the Phillies this season before having elbow ligament-replacement surgery in June.
He is 2-8 with one save and a 3.85 ERA in 97 major league appearances.
Spence, 24, made 11 relief appearances over two stints with San Diego this year. He went 0-1 with a 4.35 ERA.
A native of Australia, Spence is 0-3 with a 3.15 ERA in 51 games for the Padres. A lefty specialist, he made 40 appearances in 2011 but pitched just 29 2-3 innings.