Something fast
Edit catalog/includes/modules/boxes/bm_manufacturers.php
Search lines 65, 66 and 67
$data = tep_draw_form('manufacturers', tep_href_link('index.php', '', $request_type, false), 'get') .
tep_draw_pull_down_menu('manufacturers_id', $manufacturers_array, (isset($_GET['manufacturers_id']) ? $_GET['manufacturers_id'] : ''), 'onchange="this.form.submit();" size="' . MAX_MANUFACTURERS_LIST . '" style="width: 100%"') . tep_hide_session_id() .
'</form>';
Add below
$data .= '<a href="' . tep_href_link('allmanufacturers.php') . '">' . MODULE_BOXES_MANUFACTURER_BOX_ALLMANUFACTURERS . '</a>';
Add to language file catalog/includes/languages/english/modules/boxes/bm_manufacturers.php
define('MODULE_BOXES_MANUFACTURER_BOX_ALLMANUFACTURERS', 'All Manufacturers');
It would be necessary to polish the code but as it is something quick...
Best regards from Spain