"We know Tommy will join us for the start of next season but we'd like him here this week if possible," he told the Birmingham Mail.
The 35-year-old former Aston Villa and Celtic man was United's big summer signing but he has struggled with injury and has been a regular fixture on the substitutes bench.
His departure may be halted though due to a ligament injury to Iron top scorer Billy Sharp that may keep him out for some time.
As well as Johnson, the ambitious non-league outfit have made a move for free agent Nicky Summerbee, formerly of Manchester City and Sunderland, who joined last week.
They have also been linked with ex-England and Arsenal midfielder Paul Merson, who was sacked as manager of fellow Midlands club Walsall last week.