The 22-year-old, who played in the first half against Gillingham at Crabble last Saturday and again against Charlton Athletic last night, has penned a one-year deal.
Passley started his career with Fulham and made two senior appearances during his time at Craven Cottage.
While he was with Fulham, he also played in League Two while on loan spells with Shrewsbury Town and Portsmouth.
He then joined Dagenham & Redbridge and made forty-five appearances in all competitions for the then-League Two side.
Passley then had a brief spell with Whitehawk last season, scoring twice in thirteen games for the National League South side.