wokamoto

This plugin modifies RSS feeds and ATOM feeds as you want.
<h4>How to use</h4>
Putting the following templates into the current theme folder, then you can use the customized feed templates instead of the default ones.
<ul>
<li>feed-atom.php – Atom Feed Template for displaying Atom Posts feed.</li>
<li>feed-atom-comments.php – Atom Feed Template for displaying Atom Comments feed.</li>
<li>feed-rdf.php – RSS 1 RDF Feed Template for displaying RSS 1 Posts feed.</li>
<li>feed-rss.php – RSS 0.92 Feed Template for displaying RSS 0.92 Posts feed.</li>
<li>feed-rss2.php – RSS2 Feed Template for displaying RSS2 Posts feed.</li>
<li>feed-rss2-comments.php – RSS2 Feed Template for displaying RSS2 Comments feed.</li>
</ul>
Note: 
When these feed templates are not in the current theme folder, WordPress’s default feed templates will be used.

Feed Template Customize

This plugin modifies RSS feeds and ATOM feeds as you want.

uwcals

Enables WordPress to include thumbnail and custom field data in its RSS2 feed.
How is works:
<ul>
<li>To retrieve post thumbnails: Add “post_thumbnail=1” to your feed’s link. (e.g. “http://www.mywpsite/feed/?post_thumb=1”)</li>
<li>To retrieve custom fields: Add “meta_keys=meta_name1[,meta_name2,meta_name3,…]” to your feed’s link (e.g. “http://www.mywpsite/feed/?meta_keys=post_quote,post_contributor” will retrieve the “post_quote” and “post_contributor” custom fields’ value, if these fields have been added to the post.)</li>
<li>To further customize your feed: use any other WP Feed options, as listed on https://codex.wordpress.org/WordPress_Feeds/.</li>
</ul>
Example: 
The http://news.cals.wisc.edu/feed/?cat=67&post_thumb=1&meta_keys=academic_info will retrieve all posts in category 67, along with their thumbnails (if available) and the value of the ‘academic_info’ field (if available).

UW CALS Extend RSS2 feed

Enables WordPress to include thumbnail and custom-field data in its RSS2 feed

Feed Template Customize vs UW CALS Extend RSS2 feed