Johnsen, who was released by Aston Villa at the end of last season, is available on a free transfer and is currently on trial at Premiership side Southampton.
The Norway centre-half made 46 appearances for Villa last season, scoring one goal.
Kevin Blackwell is busy scouring the market for experienced players who are willing to accept a pay-as-you-play deal and along with Steve Guppy, Chris Bart-Williams and Craig Hignett, Johnsen could fit that bill.