You would only need to add the concat() to the select statement if you wanted to display that value. To add it to the search you can just add it to the array of columns searched. It already has supplier and description in the field list:
$searchrs_products_WADbSearch1->setSearch(array("type"=>"k", "comparison"=>"like", "join"=>"and", "and"=>" ", "or"=>", ", "end_encap"=>"'", "start_encap"=>"'"),array("products.itemnumber", "products.supplieritemnumber", "products.description", "products.german_description", "products.supplier", "products.barcode"),"c",$_SESSION["keywords"]);
So you shouldn't need to add anything to it to find results from the supplier and description fields.