how to add new currency support to golang.org/x/text/currency

Considering the Currency Unit represents ISO_4217, and seems fairly complete, you can't extend it directly.

You would need to go:

• either with a third-party package like Rhymond/go-money, which could be more complete
• implement your own definition, as in thrasher-/gocryptotrader/currency, where new currencies can be extended through a json config file.
  Or, as mh-cbon proposes in the comments, through a call within init().