Freight (non shippable Item) to be printed on Invoices for Transfer Order

we have the following requirement

1. Transfer Order is for the standard item (shippable and invoiceable)
2. There is another line needed which is non-shippable (freight) on transfer order. This has to be printed in the invoice ?

Can someone pls guide how can be achieve this. We know that we can add the Freight at the time of shipment but how can we add the same to Invoices.