How can I modify the header of RSS feed items?

this value comes from the template tag the_author().
You can also filter this.
But it is important, that you check, that the filter only work on the feeds; see the follow example at the check for is_feed().
After this i change the autor name only, if the string has the value ‘name_xyz’; here is the point that you change your data from your requirements.

I write from scratch, don’t tested!

add_filter( 'the_author', 'fb_change_fead_creator', 10 );
function fb_change_fead_creator( $text ) {

    if ( ! is_feed() )
        return;

    if ( 'name_xyz' == $text )  
        $text="my new name";

    return $text;
}

Best