submenu item edit a specific post

You do not need a callback function at all.
You could display (yet another) custom edit screen, but if the custom post type is set up correctly you should be able to reach it simply by pointing to http://yourdomain.com/wp-admin/post.php?post=9999&action=edit where 9999 is any post ID that corresponds to a post of said type.

Hence you can use post.php?post=9999&action=edit as the menu slug (see the codex page of add_menu_page for reference on parameters) with add_submenu_page and you’re good to go:

add_submenu_page(
    'parent_slug',
    'Edit My Special Post', // page title, change to liking
    'Edit SP', // menu title, change to liking
    'edit_posts', // required capability, change to liking
    'post.php?post=9999&action=edit' // menu slug, change post ID
);

The last parameter, the callback function, you don’t need at all.