The 18-year-old enjoyed loan spells at Notts County and Hereford before being released by the Potters at the end of the last campaign.
Sampson said: "Nathaniel is a young player who I think has got a lot of potential.
"He was outstanding when Hereford beat us at Sixfields last season. I have kept an eye on him since and as soon as I knew he was available I was keen to bring him here.
"He is big and strong and has a good left foot. He gets stuck in and can handle himself well.
"We need a mix of youth and experience but I am particularly keen to bring exciting young players with potential to the club so that they can learn, develop and grow together."
Source: Team Talk
Source: Team Talk