The 19-year-old Great Britain junior becomes coach Tony Hand's second signing as Phoenix prepare for their return to the Elite League in September.
Walker, from Kirkcaldy in Scotland, was recommended by former Storm coach Daryl Lipsey who looked after the Scot at Swindon Wildcats.
"Adam has enormous potential and I'm looking forward to bringing out the best in him and helping him develop," said Hand.
"At 6ft 1in, he has good size and knows where the goal is - his numbers last season alone proved that and people really noticed his performances in the World Junior Championships.
"He can play centre or on the wing and that versatility is important as we build for the future in Manchester."
Walker started his career with Fife Flyers aged just 14. He was voted best British player for Swindon last season with 29 goals and 43 assists.
He was a key member of the GB Under-20 squad at the World Junior Championships in Romania last December. He top scored with 11 goals and 7 assists as Great Britain won gold in the Division Two tournament in Bucharest.
Walker, who joins another British youngster Joe Mellor on the roster, said: "I want to play at the highest level and Manchester in the Elite League is the perfect opportunity. I had a great year in Swindon, the people there were very good to me and wanted me to re-sign.
"But, the chance to learn from the best in Tony and test myself against bigger, better, quicker, stronger players is something I want to do and I feel this move has come at the perfect time for me."
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