What is the Accounting Treatment and configuration for PO without material number

What is the Accounting Treatment and Configuration for PO without material number? My purchasing team is creating PO without material number using account assignment K and charge the expenses to cost center. But I notice the GR/IR account posted to GR transaction and IR transaction is not the same. I guess is missing configuration. Should I configure the account determination of GR and IR based on Material Group and Valuation Class? And what is the correct accounting treatment for both GR and IR?