require($_SERVER["DOCUMENT_ROOT"]."/bitrix/modules/main/include/prolog_before.php");
CModule::IncludeModule("iblock");
$arFields[0] = Array(
"NAME" => 'Название',
"ACTIVE" => "Y",
"SORT" => "500",
"CODE" => "CODE",
"PROPERTY_TYPE" => "N", // тип смотри внизу описания
"IBLOCK_ID" => $IBLOCK_ID
);
$ibp = new CIBlockProperty;
if($PropID = $ibp->Add($arFields[0])) {echo 'OK: '.$PropID;}
else {echo 'Error: '.$ibp->LAST_ERROR;};