Automatically adding categories and authors in custom menu?

You can use the filter hook wp_get_nav_menu_items to add new items in the WP Nav Menu.
The follow example is to add the last posts in the nav menu.

You can add to this filter your custom function, like to add each post from author with a specific ID. The logic to add the posts is inside your custom function, like the follow example in replace_placeholder_nav_menu_item_with_latest_post.

// Front end only, don't hack on the settings page
if ( ! is_admin() ) {
    // Hook in early to modify the menu
    // This is before the CSS "selected" classes are calculated
    add_filter( 'wp_get_nav_menu_items', 'replace_placeholder_nav_menu_item_with_latest_post', 10, 3 );
}

// Replaces a custom URL placeholder with the URL to the latest post
function replace_placeholder_nav_menu_item_with_latest_post( $items, $menu, $args ) {

    // Loop through the menu items looking for placeholder(s)
    foreach ( $items as $item ) {

        // Is this the placeholder we're looking for?
        if ( '#latestpost' != $item->url )
            continue;

        // Get the latest post
        $latestpost = get_posts( array(
            'numberposts' => 1,
        ) );

        if ( empty( $latestpost ) )
            continue;

        // Replace the placeholder with the real URL
        $item->url = get_permalink( $latestpost[0]->ID );
    }

    // Return the modified (or maybe unmodified) menu items array
    return $items;
}

The source example is from Viper007Bond, see the post for more information about the code.