You can hook into your feed with the according filters and then add your stuff.
Basically that codex page suggests (and that's still valid, you find the feed templates inside
/wp-includes/, the files start with
feed-) that you hook into
the_content and you check with
is_feed() if you need to modify the content for the feed.
To save you some hassles, you can register your whole plugin to activate on the
do_feed_rss2 (or whichever you use) hook, so you do not need to check for
The contents of magic fields (custom fields to be precise) can be read out with existing functions, you find the documentation here: Custom Fields.