When you add a compnent to a NWA one of the parameters you need to specify is when to release the reservation/purch.req.
Add your material with your freez quantity and set the Res./Pur.Req to 1-Never. This will create a plan cost for the material but will never release it to MRP or create a PR. The plan cost will be added to the project over the dates in the NWA, you'll need to see how this affects your reporting and other calculations that use plan cost.
Then, when you want to really get this material you will add it again with the correct indicator, quantity, and required date. You will have to reduce the freez quantity by the same amount or you will double your planned cost.