PrefabUtility.ReplacePrefab

Description

Replaces the targetPrefab with a copy of the game object hierarchy go.

Returns the prefab game object after it has been created. If connectToPrefab is enabled go will be made an instance of the created prefab.

// Creates a prefab from the selected GameObjects.
// Creates a prefab from the selected GameObjects.
// if the prefab already exists it asks if you want to replace it

@MenuItem("Examples/Create Prefab From Selected")
static function CreatePrefab() {
    var objs = Selection.gameObjects;

    for (var go : GameObject in objs) {
        var localPath : String = "Assets/" + go.name + ".prefab";
        if (AssetDatabase.LoadAssetAtPath(localPath, GameObject)) {
            if (EditorUtility.DisplayDialog("Are you sure?", 
                "The prefab already exists. Do you want to overwrite it?", 
                "Yes", 
                "No"))
                    CreateNew(go, localPath);
        }
        else
            CreateNew(go, localPath);
    }
}
@MenuItem("Examples/Create Prefab From Selected", true)
static function ValidateCreatePrefab() {
    return Selection.activeGameObject != null;
}

static function CreateNew(obj : GameObject, localPath : String) {
    var prefab : Object = PrefabUtility.CreateEmptyPrefab(localPath);
    PrefabUtility.ReplacePrefab(obj, prefab, ReplacePrefabOptions.ConnectToPrefab);
}