I struggled a bit with a broadly similar problem - see my plugin at  http://community.elgg.org/plugins/1524958/1.01/au-group-tag-menu for one possible and simple way of doing this. It's designed only for groups and pre-specified tags rather than a freeform search, but the general principle might work for your system.  I created my own page to present the results rather than hooking into search because the search interface itself is a bit confusing, always taking you out of the context you are in and breaking the flow.

In your options:

'type' should be 'object'

'subtype' should be 'gestore_prodotti'

Also for search you need to make sure it's registered

http://reference.elgg.org/entities_8php.html#ade3d136ebb672909d60d6e3f10095818

Thanks for your answers. I was realizing today that I made some mistakes with type (according to tutorials/blog and this guide: http://docs.elgg.org/wiki/Engine/DataModel/Entities

Now i have tested and it works using this option:

$object = new ElggObject();
$object->subtype = "gestore_prodotti";
$object->title = $title;
$object->option = $option;
$object->description = $body;

For search I'm reading readme.txt in search and as Matt suggested me, this file says:

To appear in search you must register your entity type and subtype
by saying in your plugin's init function:

 register_entity_type($type, $subtype);

I have inserted this code in start.php:

register_entity_type(object, 'gestore_prodotti');

But it doesn't effect on tag search.

I have a question about type and object... I'm looking at elgg database, according to what I'm reading on documentation I can see entity-object table with blogs, bookmarks.. Right now I have created a newelggobject of type object and subtype gestore_prodotti. Where I can see this subtype in elgg_objects_entity table?

Thanks for your support guys!

For 1.8 the function is elgg_register_entity_type()

Also make sure both the type and subtype are in quotes, otherise php may consider that a constant which resolves to null (which is probably why your search isn't working)

New objects should be new rows in the objects_entity table

Thanks for suggestions Matt. I tranformed code into

elgg_register_entity_type('object', 'gestore_prodotti');

But search doesn't work anyway. I have checked again in objects_entity table and there is no trace of a object rows (or gestore_prodotti row?).

Sorry Matt, rows are there.. but search doesn't work. Right now I have corrected search like this:

<?php

($vars['search_type'] == 'tag') {
 $tag = get_input('tag');

 $title = elgg_echo('Ricerca prodotti per tags', array($tag));

 $options = array();
 $options['query'] = $tag;
 $options['type'] = "object";
 $options['offset'] = $offset;
 $options['limit'] = $limit;
 $results = elgg_trigger_plugin_hook('search', 'tags', $options, array());
 $count = $results['count'];
 $gestore = $results['entities'];
 $content = elgg_view_entity_list($gestore, array(
  'count' => $count,
  'offset' => $offset,
  'limit' => $limit,
  'full_view' => false,
  'list_type_toggle' => false,
  'pagination' => true,
 ));
}

$params = array(
 'title' => $title,
 'content' => $content,
 'sidebar' => elgg_view('gestore_prodotti/sidebar'),
);

$body = elgg_view_layout('one_sidebar', $params);

echo elgg_view_page($title, $body);