Leadbitter has represented England from Under 16 to Under 19 level and featured in the European U19 Championship Final but finished on the losing side.
Harford today told PA Sport: "We are just trying to put a deal together for a midfielder from Sunderland and if everything goes through he should be in the squad to face Southend. We are also looking to bring in a centre-forward and a centre-half."
The BBC also reported that the Millers had missed out on QPR midfielder Adam Miller, who has decided to join Peterborough United on a 3 month loan deeal.