Two members of Neil Wood's squad, Paul Woolston and Luca Ercolani, have signed fresh contracts to continue their development with the Under-23s.
Woolston, a goalkeeper formerly with Newcastle United, appeared in the final two Premier League 2 matches of last season, while centre-back Ercolani has been at the club since 2016.
At Under-18 level, international clearance has now been received for three new additions.
Johan Guadagno is a 16-year-old keeper from IF Brommapojkarna in Sweden, while forwards Dillon Hoogewerf and Mateo Mejia have also joined.
Netherlands Under-16 international striker Hoogewerf arrives from Ajax and Mejia was previously with Real Zaragoza in Spain.