Johnson has been training at Glanford Park after watching Saturday's 1-1 home draw with Colchester.
Knill said: "We expressed an interest with Oli a while ago.
"The problem is we haven't seen him play since he was at Stockport. We couldn't make a judgment because of that.
"So we've asked him to come in and train with us this week, and then we will make a decision. If he does really well then we could sort something."