Knight, who scored 43 goals in all competitions as he claimed the Isthmian League Golden Boot during his spell with Bowers & Pitsea in 2018-19, will reunite with Cooper for a second time after enjoying success at Met Police.
Nzengo made 35 appearances in the Millers defence in 2019-20, helping the club to a record run to the quarter-finals of the FA Trophy run, scoring in the giant killing victory over Chelmsford City.
The former Aveley, Maldon & Tiptree, Thamesmead and Stanway Rovers boss has been with the club since last June but will now take the top job alongside former Leyton Orient, Brentford, Reading and Sutton player John Mackie.