Yes, you can. Provided that an array is used to define the "schema", you can use names as well as numbers to identify the subscribed fields and this also holds for extra fields.

Actually, the StockList demo front-end code is still based on the older number-based syntax and this makes it less easy to extend it. Hence, the number references to the extra fields had to be changed. Note that you should rename extra field 15 (into 115 or some custom name) as well, though it is never used with the current front-end configuration.

Moreover, as you added your custom fields in the middle of the original field list, also the numeric references to the subscribed field numbers in the "updateItem" and "onChangingValues" method may need to be changed (if you need to preserve these event handlers at all). Moving to field-name syntax surely is the best choice.

Dario