Johnstone had registered seven or more sacks in each of his previous four seasons with the Minnesota Vikings and tallied 72 sacks through his career.
Brad Badger saw action in seven games with the Raiders in 2006 and had been with the team for five seasons, netting 31 starts. He has appeared in 140 games over an 11-year career.
The Raiders signed Kris Richard, formerly of the San Francisco 49ers. A 5-foot-11, 190-pound defensive back, Richard saw most of his playing time in 2003 and 2004 with the Seattle Seahawks where he collected 50 tackles and a sack in 31 games.