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There is no adapter and it's highly unlikely that you'll break the existing shower arm inside the wall. Try inserting a philips screwdriver with as thick a shaft as will fit into the end of the existing arm. That will give you extra leverage to turn the arm to get it out. I'm sorry but Just Answer is an online only advice/consulting service. We don't make house calls.
I've replaced dozens of shower arms that did break off inside the wall without opening up the wall. Those arms basically rotted out from aggressive water they didn't break from getting unscrewed. You could also try a decent sized pipe wrench on the arm. I'm sorry but those are your only options if you want to replace the head.
I guess you'll have to call a plumber or use a bigger wrench.