There is no option to set position or order a submenu item inside an already exist main menu item.

Currently It is possible via small adjustments:-

1. Add $this->cmb->prop( 'position' ) inside add_submenu_page() inside CMB2\includes\CMB2_Options_Hookup.php
2. Also add an extra property - priority that we can use inside menu add_action hook. add_action( $hook, array( $this, 'options_page_menu_hooks' ), $this->cmb->prop( 'priority' ) );

Hope next version will add some adjustments to order a submenu.