Freight account derivation for iSupplier portal submitted invoices

We are currently implementing iSupplier and found out that when the supplier submits an AP invoice through the portal and adds manual freight lines on the invoice, the account derived on the freight lines is same as the PO charge account. We have a requirement to use the default freight account stored in the payables options -> Invoice tab screen. We could not locate any configuration options to meet this requirement. Is there a way to default freight account from payables options to the freight lines on isupplier invoices? Can we use SLAs to achieve this functionality?

Thanks

Ramesh