If we peek into the fetch_feed() function we see the instantiation:
$feed = new SimplePie();
where the object is made accessible through the wp_feed_options hook via:
do_action_ref_array( 'wp_feed_options', array( &$feed, $url ) );
where $feed is passed by reference.
This means we can adjust that object instance through the hook, before the $feed->init() is called within fetch_feed().
I searched this site for examples for you and found only one here by @Firsh. that sets $feed->force_feed(true) through the wp_feed_options hook.