Typically you do not want to be editing module OnEnters outside of for personal testing purposes unless it is absolutely required. It's terrible for compatibility purposes. For a mod that is to be publicly released, you'll want to instead rely on other methods. For NPCs that should always be present, simply adding it to the GIT's creature list is the way to go. For NPCs that need to spawn under specific scenarios, adding a trigger with a custom script would be preferable to editing the OnEnter. Or as LoneWanderer suggested, a DLG is a common vector, primarily when the NPC appears during, or as a result of, a cutscene/conversation.