Although you can create a template using the write method of nms:delivery but it's not recommended. As of now , I cannot find any API through which you can create a Message centre template and the standard way to go about this would be through the console as described here :
I know how to add new template via console, did it many times. I just want to create simple webapp for mac users to add templates without asking me for help. So inserts/updates on nms:delivery is the only way.