How to include fields as custom XML?

You can map fields to any elements defined by any module providing feed fields. If you want to include RSS elements which are not provided by any of the Views RSS family modules, you would need to define them yourself in one of your modules, implementing hook_views_rss_channel_elements() for <channel> elements and hook_views_rss_item_elements() for <item> elements (and optionally hook_views_rss_namespaces() for namespaces) (see Hooks documentation page for list of all hooks provided by the module). I haven't got to writing a detailed documentation on these hooks yet, but have a look into one of the existing modules to see how to do it (and don't forget to clear Views cache after each change, as the definitions are being cached).