adding BG fields in PO header

hi

we are using sap ehp 4. in po there is no field related to bank guarantee. i want fields related to bank guarantee at po header level.

for eg: a date field in which i can mention the validity date of BG

and a yes/no checkbox which i can check if BG is submitted on time

only these 2 fields are required and nothing much.

are there any fields in po which can utilised for this purpose? if not, then pls guide me how to proceed further