Johnson, a second-round draft pick by Minnesota in 2008, started 15 games for the Vikings in 2009 but a total of only five over the past two years. He had surgery in November to repair a torn right hamstring tendon and says he's 85-percent recovered.
Last month, the Dolphins released Bell, a starter at safety since 2006. They haven't ruled out re-signing him.
The Dolphins also announced Thursday that they have re-signed reserve linebacker Austin Spitler.