RSS Feed URL: How to remove category and add feed.xml

Hello,
I am trying to remove category and add feed.xml from the feed URL.
From
http://local.wp.com/category/baby/feed/
To
http://local.wp.com/baby/feed.xml

I am using “WP No Category Base – WPML compatible” to remove Category which is working.
http://wordpress.org/plugins/no-category-base-wpml/

I read this page http://codex.wordpress.org/WordPress_Feeds
But I have no idea how to change /feed/ to /feed.xml after I could remove the category part.

Please help!