Hi Venkateswaran -
There are two ways to do this either with user exit (MM06E005 ) or BADI - ME_GUI_PO_CUST. You need to choose one of them.
Here are some links for the same.
**************** - Adding custom fields to the Purchase Order transaction ME22N
In the above link it has been described about Line Items , As your requirement is header , you need to take the exit
EXIT_SAPMM06E_006
EXIT_SAPMM06E_008
Here is another link which will on the same.
Also for the BADI - here is step by step process.
**************** - Step by step guide on adding a new tab in ME21N using a BADI
Let us know if you need any further help.
Thank you.
Regards,
Atul Mohanty