The 25-year-old Scot moved to the United States from Hearts in 2017, initially joining Minnesota United.
"Sam is a great dribbler who can both create and score goals," boss Ben Garner told the club's website.
"He's a dynamic player who can play on either side of the pitch & is very versatile.
"He's got a great mixture of experience, having played at the highest level in Scotland and also in America. We hope he can have a big impact."