To add new currency go to Setup -> Finance -> Currencies and click New Currency.
If this is your base currency you will need to mark it as base by clicking on the Star icon.
This currency will be used all over the system by default.
Pixie CRM by default record all transactions (invoices,estimates,expenses,proposals) in the base currency.
Changing the currency while recording a transaction is not possible.
If you need to record the transaction in different currency you need to set customer currency.
