I can’t actually think of an appropriate time and place for this, so if you know of one, please share. But the point is, if you want to create a default excerpt to be used on all new posts, here’s how to set that up.
add_filter('default_excerpt','setup_default_excerpt');
function setup_default_excerpt($excerpt) {
$excerpt .= 'Put your default excerpt here.';
return $excerpt;
}