Freight Charges after doing GR in PO

Dear All,

We have not entered any Freight charges in PO by mistake, And for that PO, already we have done GR also. Now how can add the freight charges to that PO ? Now the freight charge condn type in PO became as Grey.Can make it as editable after GR also ? If i did so, what will happen ? If that is not the right way, then what is the other way. I want to add this freight charges to material cost.

regds,

CB