To add Billing Plan at Sales Order Header Level

Dear All,

Our requirement is such that for a project, we need to define a billing plan at header level of sales order.

As of now we are defining billing plan at sales order line item which are having account assignment to WBS elements.

Pl. let me know whether we can add billing plan at SO header level and if it is possible then how to do it. Because right now there is no tab available for "Billing Plan" at SO header level.

If we can add it then will it be valid for all item levels?

Need your help.

Regards

Ratna Palit