This was from this very day last year. Don't think the rules have changed.
http://www.independent.co.uk/sport/football/transfers/free-agents-transfer-news-players-premier-league-clubs-can-still-sign-window-deadline-a7219781.html
I'm not sure, but I think if you leave a space free you can fill it later on. Clutching at straws anyhow.
Edit: Nope this is bollocks. Free agent u21 or we "promote" James Tilley to the bench. :lolol: