Per standard functionality the line number is a sequence number starting from 1 for records having the same requisition header id order by item_description and quantity.
In other words, the po_requisition_lines table is ordered by item_description and quantity for the same header_id and then the line_num is updated with a counter that increments by 1. It is not possible to populate line number in the interface table.
And we have the following ER for the same issue,
Enhancement Request (ER) Bug 3906591 : REQ IMPORT LINES ARE NOT COMING IN SEQUENCE
Enhancement Request (ER) Bug 4205941 : SORT ORDER OF REQUISITION LINES AFTER REQUISITION IMPORT PROCESS
Documented in MOS note - "How to Import the Requisition ordered by Line Number ? (Doc ID 1066916.1)"
Many thanks for the response..