Add custom profile info into Feed

Check out either the the_content_feed and the_excerpt_rss filters or use the regular content and excerpt filters with the is_feed conditional function.

The following should get you on the right track:

function wpse_140401_add_to_feed( $content )
{
    $twitter_handle = /* grab handle */;

    if ( is_feed() ) {
        $content .= '<p>' . $twitter_handle . '</p>';
    }

    return $content;
}
add_filter( 'the_excerpt_rss', 'wpse_140401_add_to_feed' );
add_filter( 'the_content_feed', 'wpse_140401_add_to_feed' );

Note that given the fact that the above uses filters applied to feeds only, the conditional isn’t strictly necessary.