Item Charges and Subcontract PO Lines NAV2016

We have an issue where NAV won't let us assign Item Charges to Subcontract PO Lines. It gives an error if the PO Line/receipt has a Work Center assigned to it, which of course Subcontract PO's do. I understand that this is not a PO line for an Item, but there is freight paid in the process that we want in the multiline Production Order/Purchase Order, and Item Charges would be helpful. Has anyone found a workaround for this, besides painfully computing this and add then consuming it on the Prod Order?
Thanks,