Hello there, @gilad.
Currently, the option to add the SKU column in the purchase order is unavailable. Although, you can enter the SKU number in the Description field for tracking purposes.
I've added this article for your future task about customizing your sales forms: Customise invoices, estimates, and sales receipts in QuickBooks Online.
Please don't hesitate to let me know if you have other questions or concerns. I'm here ready to back you up. Take care.