Hi,
You have two options, one option is FX differences are not posted to WBS-elements, but elsewhere with default account assignment. Usually that is a Finance Cost Center.
If for any reason you want them on individual WBS-elements the settlement process can be configured with allocation structure and particular accounts can be settled to Cost Center.
Also from AuC to the Final Asset you can select a line items settlement and settle specific accounts manually to Cost Center.
Regards,