As of Moni v3.0, this is all handled within the app!
Go to the account settings page and edit its currency. You can change it anytime.
Your overview will continue to show transactions from all accounts based on your default currency (which you can change in your main settings).
We even do the conversion for you! Exchange rates are updated once per day so they may not always show the penny-perfect amount, but can give you an idea of your overall status!