Add item to the woocommerce dashboard menu from a plugin

try this code. Paste in to function.php

add_action('admin_menu', 'custom_page');
function custom_page() {
    add_submenu_page( 'woocommerce', 'Name of page', 'Name of item', 'manage_options', 'custom-slug', 'custom_page_content' ); 
}
function custom_page_content() {
    echo 'Page content';
}