Yes, you can adjust the value of a stock item without adjusting the stock quantity. Here is an example:

This item has a COGS Average Cost of $69.73429:

Using QODBC, I adjusted the opening balance equity cost of the stock item by $250.00 using the following SQL insert statement:

INSERT INTO "InventoryAdjustmentLine" ("AccountRefFullName", "TxnDate", "RefNumber", "Memo",
"InventoryAdjustmentLineItemRefFullName", "InventoryAdjustmentLineValueAdjustmentNewValue", "FQSaveToCache")
VALUES ('Opening Bal Equity', {d'2007-05-23'}, '110', 'Value Adj 250', 'Wood Door:Exterior', 250.0, 0)

This item now has a COGS Average Cost of $106.69857:

The value adjustment details can be seen in the Inventory Valuation Detail: