Shombe, who came through the local Kinetic Academy, has penned a 12 month pro deal with Saints.
"I'm buzzing, I can't wait to get started." said Shombe, "I've been in training and I'm just ready to hit the ground running."
"The football is very different to Men's football, but I feel like I'm getting the hang of it and the players are all nice so it's going good."
Academy Director Andy Goldie also told the club's website: "We are really pleased to welcome Khiani from Lewisham Borough after a successful trial towards the end of last season.
"He’s another who comes from a long list of successful graduates from Kinetic Academy, so we know he's been developed with good values and standards which has been evident since he arrived at Staplewood.
"He already has over thirty senior games under his belt and will use that experience to compete with our Under-18s and Under-21s goalkeepers this season."