Automatically attach document to material in PO without refernce PR.

Hi All,

Currently DIR of material of line item of a PO is fetched when it has reference PR but it doesnt show for line item without PR,so how to

automatically attached DIR of material of a PO (ME22n or ME21n) when it doesnt have reference PR.?

I found a badi DOCUMENT_MAIN01 which triggers aftr click 'Attach' Button..could any one please suggest how to implement it.?

Thanks,

Abhijeet.