Purchase Requisition Message Mapping

Hi,

I'm trying to retrieve PR records using OData service from ERP and send the messages to third party system.

Here're issues that I'm facing to :

1. schema of each record is not the same. For example, PurchaseRequisition "100000" include 5 fields, but PurchaseRequisition "100020" includes 8 fields.

Question : How could I make the schema in consist?

2. How could I use message mapping to convert the structure of message from Tree to Flat?

Thanks in advance