org.joda.money.CurrencyUnit.registerCurrency is currently package-visible, which means the only ways to register a currency is by either placing org/joda/money/MoneyDataExtension.csv on classpath or using a completely new currency provider.
The latter is no possible for a library. The former is not a very good idea because it potentially prevents apps from using this extension mechanism (two resources with the same name of classpath is a famous source of head scratching and heisenbugs).
I can't think of a downside to making registerCurrency public, please consider making it so.
Allow registration of currencies by application code