Add custom left nav item to Genesis primary menu.

functions.php

<?php

add_filter( 'wp_nav_menu_items', 'custom_nav_item', 10, 2 );
/**
 * Callback for Genesis 'wp_nav_menu_items' filter.
 * 
 * Add custom left nav item to Genesis primary menu.
 * 
 * @package Genesis
 * @category Nav Menu
 * @author Ryan Meier http://www.rfmeier.net
 * 
 * @param string $menu The menu html
 * @param stdClass $args the current menu args
 * @return string $menu The menu html
 */
function custom_nav_item( $menu, $args ){
        
    // make sure we are in the primary menu
    if ( 'primary' != $args->theme_location )
    	return $menu;
    
    // see if a nav extra was already specified with Theme options
    if( genesis_get_option( 'nav_extras' ) )
            return $menu;
    
    // additional checks?
    
    // append your custom code
    $menu = sprintf( '<li class="left your-custom-code-class"><span>%s</span></li>', __( 'Your custom code' ) ) . $menu;
    
    // return the menu
    return $menu;
        
}