You can allow the use of multiple currencies in your company by clicking the Currencies button on the Settings page. This is irreversible, so you will be asked for confirmation.
If you have already enabled multiple currencies in your company, clicking Currencies on the Settings page lets you add new currencies and set custom exchange rates.
To set a custom exchange rate for a currency, click "Edit exchange rate" in the corresponding row. Here you can also set whether this rate value will expire, and when.