When shipping orders with FedEx it is possible to bill duties to another account number, to achieve this you will need to add the Account Number to Extra Code 4 of the required FedEx service(s) and add THIRD_PARTY to Extra Code 1:
Click Shipping Management then Extras.
Click Courier Service Codes.
Find the FedEx service then click Edit.
Add THIRD_PARTY to Extra Code 1.
Add the Account Number in Extra Code 4.
Click Update.