Cusanu brings versatility to the Robins forward line, given his ability to play on either wing, as a number 10 or as a central striker.
His arrival at Hurst Cross comes after Kielen Adams, Nathan Lowe and George Milner signed new deals in recent days.
Manager Steve Cunningham said: "I have been a fan of Marcus for years and I'm looking forward to seeing him in an Ashton United shirt.
"He will bring an attacking threat with goals and assists, but also a work rate that is second to none."
Photo credit: Ashton Athletic
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