The 22-year-old former West Ham youth player tried to earn a deal in the summer.
He played in a number of the club’s pre-season friendly games but didn’t extend his stay.
He gets a second bite at it and will wear the number 14 shirt after becoming Garry Hill’s second signing.
“Freddie’s been very impressive in training,” his new manager said on the club’s official website.
“As everyone knows we’ve been very much down to bare bones in the past week or two and we go into an intensive period of games now so Freddy brings us up to an 18-man squad.”
The son of former Spurs and West Ham man John Moncur, he could play on the FA Trophy on Saturday against Dagenham.