You probably can but why would you? CDMA is a superior system in my experience. If the carrier won’t (or can’t) support CDMA you would be better off finding one that does in my opinion. I’ve had two phones that I converted to CDMA from GSM and both immediately had MUCH better signal strength and better service and voice clarity. From what I’ve read, the only real need for GSM is for folks traveling outside the USA frequently.