A button or context menu option on the character sheet that will translate as much information as possible between PC and NPC. This would allow CKs to build threats that have class levels using the regular PC character sheet and quickly translate them into NPC objects that can be used in Encounter objects.
Also, on the rare occasion where an NPC gets taken over by a player, a new PC could be generated. This would allow the player to control the NPC using a more familiar interface or even take over the converted character as a new PC.