The 18-year-old full-back joins on a free transfer, having most recently featured for the first-team of Southern League Division One Central side Hertford Town.
Musanhi initially featured on trial for the Under-18s last season, playing in back-to-back wins against Norwich City and West Bromwich Albion which kick-started a strong end to the season for Rob Quinn’s side.
He will now slot into Darren Powell’s Under-21s side this season, as they prepare for the new Premier League 2 campaign.