WebServices - Linking Vendor Bill to PO

Hi,

We have to import the historical Purchase Orders and Vendor Bills into NetSuite using Web Services. Can someone please let us know how to link Vendor Bill to a PO? We don't see any PO # on the "Vendor Bills" API.

For e.g., We import 5 Purchase Orders of a Vendor using Web Services and did the receipt confirmation (WSDL V2.6). We have to enter all the bills of those purchase orders. We notice that the "Vendor Bill" API do not have any link to the Purchase Order - This is required as we need to change the status of the Purchase Order from "Pending Bill" to "Bill entered". In GUI, we use the screen "Bill PO" which links the Bill to the PO.