Add a line to feed-rss.php with plugin or function

I use a lot of custom fields in my posts, and I do it by including the following line below my content in my posts.

<?php include( TEMPLATEPATH . 'aggregate.php' ); ?>

Since custom fields don’t work in RSS feeds, I managed to get around this by including the custom fields line above in the feed-rss2.php file here:

<content:encoded><![CDATA[<?php the_content() ?><?php include( TEMPLATEPATH . 'aggregate.php' ); ?>]]></content:encoded>

I’d like to be able to do this in my theme functions file, or with a plugin, so that I don’t modify core WP files. How is this possible? Thanks!