No, you cannot create another index, but you could concatenate the SKU on the name:
$AuthNet_itemized[$nextIndex] = "Name";
$AuthNet_itemized[$nextIndex] = "SKU:".$WA_Store_Cart->DisplayInfo("SKU") ." Name:".$WA_Store_Cart->DisplayInfo("ID") ."";
The order number in power store is based on the ID of the record created in the order table. The record is not stored in the database unless the order is success full, so the ID sent to Authorize.net cannot be changed. However, the session ID is stored in the orderReferanceID column of the orders table. This column can be used to match the Authorize.net order number with the order in the database.