I am performing PO Conversion from legacy system, i need to bring in same "header id"
which is on legacy PO Header and Line, is it right if i populate interface_header_id column with Legacy "header id" .
You can. Just make sure that you don't have other records in the interface table with the same interface_header_id (and probably same interface_source_code).
Also you need to make sure you update the value of po_headers_interface_s sequence used to generate the interface_header_id.
But if your purpose is to use/view the old header id after you bring the PO to Oracle, this won't help. The interface_header_id data does not come over to po_headers_all.
You may consider creating a DFF on PO.