Campbell was the New York Jets' sixth-round pick in the 2013 draft but didn't play a down. He was cut before the start of this season.
Towns is an undrafted rookie free agent out of Samford College, who was released by Washington in August.
The Bills (3-2) have needs at both spots in preparing to host New England (3-2) on Sunday.
AP NFL website: www.pro32.ap.org and http://twitter.com/AP-NFL