Appau is a midfielder who joined the academy two years ago having been released by AFC Wimbledon.
He featured in Surrey Senior Cup games for the Cards last season and has been training with the first-team during pre-season, playing in a number of the friendly matches.
Nsimbi, a striker, and Mason, a goalkeeper, have also been training with the Cards during pre-season, with Nsimbi coming off the bench in a few of the friendly matches.
Manager Anthony Limbrick said: “It’s a testament to the academy that they have produced three players who are now part of the first-team squad. Declan, Sam and Ivan all performed well during a tough pre-season. Credit to Scott Harrison and Jane Spong for their hard work throughout the academy and community programme.”
Woking`s academy manager Scott Harrison added: “It’s great to see a manager putting faith in our academy system and trusting these young players within the first-team environment.
“Ivan and Sam have potential but this time with the first-team will gain them vital experience.
“I’m happy for Declan as he was released by AFC Wimbledon and we took him on two years ago, I saw potential in him then and each year he has progressed, he still has lots to learn, but like all three I’m proud, like all of the staff are, to see this progression and I hope we can continue to find more prospects like these and support Anthony’s needs”.