$arFields = Array(
"NAME" => "Внешний ID элемента",
"ACTIVE" => "Y",
"SORT" => "600",
"CODE" => "ExtElementID",
"PROPERTY_TYPE" => "E",
"IBLOCK_ID" => 2,
"LINK_IBLOCK_ID" => 1
);
$ibp = new CIBlockProperty;
$PropID = $ibp->Add($arFields);