Check out how activity RSS feeds are parsed in:
/wp-content/plugins/buddypress/bp-activity/feeds/
There are a few hooks you could potentially use to add additional content.
If you want to filter the item description, use the “bp_get_activity_feed_item_description” hook available in the bp_get_activity_feed_item_description() function.
Various other filters are available for the item title, etc. in bp-activity-template.php as well.