Where can I add core  elements with Views RSS
It sounds like you're missing a couple things. First, make sure that you have the Views RSS: Core Elements submodule enabled -- this is what allows you access to the many other fields. This may already be enabled and if so, the reason you're not seeing the fields is that you need to switch your format to RSS Feed - Fields, ex:

 

by clicking the 'RSS Feed' link in your Format Settings. 

Now, when you adjust the settings for this, you'll see the options for the item elements you were looking for. 



You'll need to add the appropriate fields from your content to have them appear in the select lists.