Add custom class to existing menu items from custom meta

I’ve got the filter just for you: nav_menu_css_class

function wpse_175057_nav_menu_css_class( $classes, $item ) {
    if ( $item->type === 'post_type' && $class = get_post_meta( $item->object_id, '_icon', true ) )
        $classes[] = $class;
    return $classes;
}

add_filter( 'nav_menu_css_class', 'wpse_175057_nav_menu_css_class', 10, 2 );