The Academy duo agreed terms on one-year deals with the Championship club after impressing for the youth sides during the current campaign.
Defender Fenwick told the club's official website: "It's a big day for myself and my family.
"I've worked really hard and now I want to push on towards the first team and learn from the senior pros."
Dieseruvwe, who plays in attack, added: "This is what I've been working towards for the two years I've been at the club so I'm really delighted.
"My hard work has paid off so far because I have reached this point but now I have to get my head down and work even harder."