Normally a retailer will always send orders based on 1 currency, or send a currency code on the orders. In this case you can simply turn on the 'Uses Currency' settings for the accounting systems that support multi currency.
If the currency is not being sent, but is required then you can use the Supplier Currencies screen.
You can supply the currency code to use, the Lambda expression (this is based on the Document), the property is compare on the Lambda expression, and the value to match.
The example below shows a simple Lambda expression which is "Use NZD when the Document SenderID is equal to 'TPID'"
Contact support for more help on this if required