How to add a new language code to an existing language pack?

How to add a new language code to an existing language pack?

I want to add a few more codes and translations manually without scanning the website, is this possible?

Adding more language codes to an existing one?
Are you asking to add pt-BR alongside with having pt. Or rename xx to yy?

When you say scanning I think you mean adding more translation keys. How to add more? Execute the code language::translate(key, Injection). Or import using CSV under Admin -> Translations, or add them directly to the database table lc_translations.