Gareth Ainsworth has handed an initial one-year contract to Luke O'Nien after the midfielder impressed during a trial period with the club.
The 20-year-old was released by Watford in the summer and joined the squad for the trip to France before scoring in the 3-1 victory at Wealdstone, where he made 35 appearances during a loan spell last season.
Ainsworth said: "Luke came to us highly recommended and he's proved himself to be a very talented player who will be an excellent addition to the squad.
"He's still young but his time on loan at Wealdstone gave him a great introduction to senior football and I'm looking forward to working with him to unlock more of his potential."
O'Nien joined the Vicarage Road club at the age of nine and progressed through the ranks to make his first-team debut in March 2014. During his time with Wealdstone, he proved his versatility by also occupying the right-back spot when required, and contributed five times in total during the 2014/15 campaign.
He becomes Ainsworth's seventh capture of the summer and takes the number of registered players to 18, which Ainsworth says may be his limit.
"Having a small squad worked well for us last year and I think 18 is a good number which we can then supplement with loan players, like we have done with Michael Harriman.
"There are still a few positions I'd like to add to and we're in discussions with a number of clubs regarding the possibility of bringing their players to Adams Park where we can help them with their development on and off the pitch."