Goto
Appearance -> Menus
You can see all categories listed here including wp-ecommerce categories.
Just create a new menu for your wp-ecommerce categories. Lets call it “ecomcategories”. You can call this menu in your theme using this code.
<?php wp_nav_menu( array('menu' => 'ecomcategories' )); ?>
You can just drag and drop for sub menus.
You can create unlimited menus and submenus.