If you're a module author and want that module to provide extra messages for your project, you can merge them into the normally loaded messages by using the i18n:extend-messages hook.
To do this, in your module's setup file listen to the Nuxt hook and push your messages. @nuxtjs/i18n will do the rest.
This is particularly useful if your module use translated content and you want to offer to users nice default translations.
Example:
export default function () {
  const { nuxt } = this
  nuxt.hook('i18n:extend-messages', function (additionalMessages) {
    additionalMessages.push({
      en: {
        'my-module-exemple': {
          hello: 'Hello from external module'
        }
      },
      fr: {
        'my-module-exemple': {
          hello: 'Bonjour depuis le module externe'
        }
      }
    })
  })
}
Now the project has access to new messages and can use them through $t('my-module-exemple.hello').