BAdi's for Preq's and Contracts

Iam trying to find badi's for custom fields in purchase requisition(me51n and me52n) in header level and custom fields in contracts(me31k and me32k) in header level.If don't is there any user exits for those one's.once i developed the screen how to configure to add that screen to the badi.Can any one explain clearly.

Avinash m.