New arrival Rumarn Burrell will go straight into the Academy and be under the tutelage of Craig Liddle.
Burrell, 18, was born in Birmingham and joined City's youth set-up before moving to Grimsby.
A striker, he was top scorer for the Mariners U18s in the 2017/18 season, progressing to play with their reserves last season.
He made his first team debut towards the end of last season, going on to make four appearances in League Two.
A strong and pacey frontman, Burrell, who turns 19 in December, signed his first pro contract in April this year.