Try adding the concat to the array instead of the select like:
$searchrs_products_WADbSearch1->setSearch(array("type"=>"k", "comparison"=>"like", "join"=>"and", "and"=>" ", "or"=>", ", "end_encap"=>"'", "start_encap"=>"'"),array("products.itemnumber", "products.supplieritemnumber", "products.description", "products.german_description", "products.supplier", "products.barcode", "CONCAT(supplier, description)"),"c",$_SESSION["keywords"]);