@Jort de Vreeze I know about that in Sale" but I want the same thing in "Purchase" module.
Yes you can, and whether or not it affects your inventory valuation depends on a few things: Is it a storable product? If it is and your valuation method is Automated then Odoo will incorporate the total value for the purchase into your inventory valuation (at whatever method you are using AVCO, FIFO or standard cost) when the product is received.
In my opinion, best practice it to reflect what occurred in real life in your ERP system. If your vendor is giving you free product that you will stock and potentially resell, and then there is nothing wrong with receiving the product at zero cost. However, its important to understand how that will affect your costing. On AVCO the zero cost is incorporated in, so your Average Cost per unit will go down. On FIFO, you'll have a zero dollar cost layer. If you're okay with that, then you can certainly receive the product at zero cost.
If you're not okay with that, create the PO with a price and then issue a credit note to offset the related bill.