Hi varun,
Use MM06E005 enhancement to add fields in screen of ME21N
SAPMM06E 0101 CUSTSCR1 SAPLXM06 0101 Subscreen: PO header
SAPMM06E 0111 CUSTSCR1 SAPLXM06 0111 Subscreen: PO item
SAPMM06E 0201 CUSTSCR1 SAPLXM06 0201 Subscreen: agreement header
SAPMM06E 0211 CUSTSCR1 SAPLXM06 0211 Subscreen: agreement item
SAPMM06E 0301 CUSTSCR1 SAPLXM06 0301 Subscreen: RFQ header
SAPMM06E 0311 CUSTSCR1 SAPLXM06 0311 Subscreen: RFQ item
Hope this helps,
sivaganesh.