Purchasing Policy : How it works

Hi,

I have edited and PurchreqLine and Purchline and added a field in each table. I also updated the class which converts the purchase requisition to purchase Order and successfully mapped my custom fields. 

The problem is that when we create a purchase order using a Purchase policy then the custom field is  not mapped and the purchase order is created without it. Can any one tell me which class I have to update in order to map my custom field when PO is created using the policy.

Thanks.