digitalgreg

Hi, I found the solution myself.
I’ve downloaded the database and found out that TheCartPress stores the sku as a custom field in postmeta table.

I’ve came across how to add custom fields to search results and I did.
You need to modify fuctions.php with the following code:

add_filter('posts_where', 'advanced_search_query' );

function advanced_search_query( $where )
{
  if( is_search() ) {

    global $wpdb;
    $query = get_search_query();
    $query = like_escape( $query );

    // include postmeta in search
     $where .=" OR {$wpdb->posts}.ID IN (SELECT {$wpdb->postmeta}.post_id FROM {$wpdb->posts}, {$wpdb->postmeta} WHERE {$wpdb->postmeta}.meta_key = 'tcp_sku' AND {$wpdb->postmeta}.meta_value LIKE '%$query%' AND {$wpdb->posts}.ID = {$wpdb->postmeta}.post_id)";

    if(WP_DEBUG)var_dump($where);
    }
    return $where;
}

Hope that will help somebody else