There is another way to do this if you don't want the accountants to be able to manually update the automatically imported FX rates in the system. You can make the field editable using a workflow on just a selection of transaction types
Scope of our workflow is Transaction (Customer Deposit, Customer Payment, Customer Refund, Journal Entry, Vendor Payment)
And state 1 does SET FIELD DISPLAY TYPE = NORMAL before record load
This still works even if the role has view permission only on Currency.