Replace threadless headset retained bearings with sealed bearings?
Yes, it is possible. The headset compatibility is determined by steerer tube and head tube dimensions. The type of bearings inside does not really matter, and the difficulty of getting spare parts for sealed bearings or maintaining loose ball bearings are both exaggerated. 

When replacing the headset, check the condition of the frame: if the creaking is because of deformed head tube, a new headset will not help.