Former Hearts and Hibernian midfielder Stewart, 29, who made a handful of appearances for Manchester United after arriving from Rangers in 1998, and the mystery Frenchman will take part in a behind-closed-doors friendly alongside new goalkeeper David Gonzalez.
Simon Grayson said: "We've got a couple of lads up here on trial who will play [against York City at Thorp Arch]. As ever, we'll have a look at them over the next few days and see how things go."