Line items are different between IDOC and PO

Hi

How can I replicate (create) PO with the same line items that are there in IDOC?

Scenario is...

PO will be replicated from one SAP to another SAP using PORDCR1 message type

Line items in the IDOC are 11,12,13,21,22,23 (all these are main items)

When I process this IDOC, line items in PO are coming either 10,20,30 or 1,2,3 but i need as 11,12,13.

Can you tell me how can I replicate PO with external line item number.

Thanks in Advance.