The inventory balances are calculated by QuickBooks based on all of the transactions the use the items. This includes purchases and sales and also inventory adjustments.
So, in effect it's the tables of transactions that has to be impacted - by recording transactions.
Inventory adjustments can impact Quantity on Hand, Value, or Both. Recording them may be a way to get what you want.
If you want to import them in batches instead of manually recording them, you can use our BRC IIF Transaction Creator Pro to do that.