The game will take place at the Don Valley Stadium in Sheffield, where the Millers play after leaving Millmoor, on Wednesday July 27. Kiick-off will be at 7.45pm.
It is the third friendly confirmed by the iron, with games at Winterton Rangers (July 12) and Halifax Town (July 16) already on the schedule.
Knill first took charge of Rotherham in January 2005 for 11 games as caretaker boss, returning to the job in December 2005 until March 2007.