Here now produce a WordPress category drop lower menu, than the category menu display the default category list, but require a custom filed ought to be display within this menu list. Now i personally use this code for category menu.

<?php 
    wp_dropdown_categories( array(
   'name' => $this->get_field_name( 'category' ),
   'selected' => $instance["category"],
   ) );
?>

Here' need give a custom menu option "all".

<select class="postform" id="widget-categoryposts[3][cat]" name="widget-categoryposts[3][cat]">
    <option selected="selected" value="1" class="level-0">Uncategorized</option>
    <option value="18" class="level-0">thumb</option>
    <option value="19" class="level-0">snake</option>
    <option value="all" class="level-0">all</option>
</select>

Technology-not only in by doing this:

<?php 
wp_dropdown_categories( array(
    'name' => $this->get_field_name( 'category' ),
    'selected' => $instance["category"],
    'show_option_all' => 'All'
) );
?>

In which the "all" option will seems using the text you specify within the 'show_option_all' element along with the value "" within the Drop Lower area.