Item fulfillment package list?

I am trying to create an item fulfillment based on a sales order, but I cannot find how to set the package list on the item fulfillment record. This is the code I'm trying, and I'm just guessing at package list field names, since they seem to be undocumented:

 // Create new item fulfillment based on the given sales order  var itemFulfillment = nlapiTransformRecord('salesorder', recordId, 'itemfulfillment');  itemFulfillment.setFieldValue('trandate', nlapiDateToString(new Date()) );    // Copy line items to fulfillment  var itemCount = order.getLineItemCount('item');  for (var i = 1; i <= itemCount; i++) {      itemFulfillment.insertLineItem('item', i);      itemFulfillment.setLineItemValue('item', 'item', i, order.getLineItemValue('item', 'item', i));      itemFulfillment.setLineItemValue('item', 'quantity', i, order.getLineItemValue('item', 'quantity', 1));      itemFulfillment.commitLineItem('item');  }    // Add package list  itemFulfillment.insertLineItem('packagelist', 1);  itemFulfillment.setLineItemValue('packagelist', 'packageweight', 1, 1.0);  itemFulfillment.setLineItemValue('packagelist', 'packagedescr', 1, 'UPS Ground');  itemFulfillment.setLineItemValue('packagelist', 'packagetrackingnumber', 1, trackingNumber);  itemFulfillment.commitLineItem('packagelist');