Opoku, 26, has been with Carlton for four years and has built a reputation of being quick and aggressive with an eye for goal.
In 38 appearances for the Millers last term in the NPL Division One Midlands, Opoku found the net 14 times, finishing as the club’s joint leading scorer alongside Nat Watson..
Opoku is moving from Nottingham to the Manchester area and former Matlock striker Darryl Thomas, now on the coaching staff at Carlton, has advised Opoku that a move to Matlock would be beneficial for him at this stage of his career.
Phillips said he has had Opoku on his radar for some time and moved quickly when he found out he would be leaving Stoke Lane to head to Manchester.
“We’ve watched him and he’s big, strong, quick and powerful with bags of goals in him,” said Phillips.
“He’s been a good servant to Carlton in the league below but he’s moving to Manchester and it’s too good an opportunity to miss for we need to add goals to our squad.
"Darryl Thomas speaks very highly of him and very highly of Matlock. We’re delighted to bring him to the club.”
The move is subject to approval by the FA and the Northern Premier League.
Phillips is busy looking to add further to the striking department and other positions in readiness for the new NPL Premier Division season which begins on Saturday, 13 August.