Soulver gets it's currency rates courtesy of Yahoo Finance.
Click Currencies in the Numbers window to see all the supported currencies in Soulver, and their rates in your base currency.
Update your currency rates by clicking "Update Rates".
Currencies will update automatically every hour.
The Answer Palette can perform multiple currency conversions for you automatically.
Click the little heart next to a currency to make it a favourite.
In order for the Answer Palette to show currency rates, you need to enter in a currency in your expression. You can do this using a currency code (like USD), or a currency symbol (like $).
Notes
All supported currencies are displayed in this pane. You cannot add new currencies to Soulver, however you can disable/enable currencies by clicking the checkbox next to their name.