mbecher

This plugin will extract the large version of an image attached 
to a post and put it in front of the content in the RSS feed. 
This might be useful if you have an image (and only a single one) 
attached to a custom field and it won’t display in the feed otherwise.

RSS Custom Fields Images

Puts large sized image attached to posts in front of content in rss feeds. Useful for images in custom fields.

گناهکار

this plugin adds post custom fields in rss feed like this:
<pre><code><item>
...
<customfields>
<key1>value1</key1>
<key2>value2</key2>
...
</customfields>
...
</item>
</code></pre>

Custom fields in RSS

This plugin adds post custom fields in rss feed.

RSS Custom Fields Images vs Custom fields in RSS