Reid, 20, has agreed a one-year deal with the club holding an option to extend it for a further 12 months.
He had loans at Barrow and Walsall last season and made four substitute appearances for Blues last July.
Moroccan striker Ahadme, 20, has yet to play a first team game for Norwich.
He played twice for the club's Under 21 side in the EFL Trophy in 2019 and scored a hat-trick for Pompey in a 5-2 pre-season win over Havant and Waterlooville.
"We've seen a lot of Gassan for Norwich U23s - and we liked what we saw," Portsmouth boss Danny Cowley told the club website.
"But we wanted to have a closer look at him with our own eyes and he's been excellent for us in training.
"Jayden is an exciting young player with a lot of potential. He's a good dribbler and has pace, athleticism and energy," added Cowley.
"Also, which is important for us, he has a love for the game and it's been enjoyable working work with him over the past week or so."
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