The objective is to fetch fields associated with a specific post id to create a custom rss feed.
It is required to use functions like the_content_feed('rss2') , the_excerpt_rss() etc outside the loop to fill the rss template.
How to accomplish this (use such functions outside the loop)?
you can create your own custom rss template and create thr loop inside it so you will have easy access to loop only function
take a look at this answer that shows how to create your custom rss How to create additional full text RSS feeds