Add support for two fields:
- Original currency type (already exists in database but has no interface)
- Original currency amount
The total amount, net amount, and fee amount should remain in a single currency system-wide so queries and reports are still useful.
Add support for currency conversion at the current rate (or maybe even old rates the system stores). I suggest a cron job tie-in that updates the rates periodically. Rates can be downloaded in XML form from the ECB: http://www.ecb.int/stats/exchange/eurofxref/html/index.en.html
If a user enters only a non-default currency, the system should convert the amount to the system-wide currency and use it to populate the total_amount field.