Purchase Info. Record with "Reason Codes" - Question?

Hi Experts,

We have a business requirement.

Client wants to have have a field in Purchase Information Record with list of reason codes.

Whenever there is a change in Price either by increase or decrease they want to select the appropriate predefined set of reason codes.

They also want to generate a report out of it.

Modifying SAP Standard PIR is possible? If so how this can be done? What will be impact of this develpoment?

Thanks

RS