to pose as; to (successfully) impersonate; to pretend to be; to pass oneself off as; to disguise oneself as
to (completely) become; to (fully) turn into
彼は別人に成り済ました。
He impersonated someone else.
犯人は医者に成り済まして病院に侵入した。
The culprit impersonated a doctor and sneaked into the hospital.
男は親戚に成り済ます手口で高齢者を騙した。
The man scammed elderly people by posing as a relative.