Read this on the main serverless docs site

Extending and overriding configuration

Plugins can extend and override the internal configuration.

To do so, plugins may use the serverless.extendConfiguration(...) method. This is only allowed at pre-init stage of serverless. The method also takes care of resolving all variables in the given value. But it does not validate you input nor the target. Improper usage can cause serverless to fail.

The serverless.extendConfiguration(configurationPathKeys, value) method takes two arguments.

If configuration in configurationPathKeys does exist the value will be overwritten. If configuration in configurationPathKeys does not exist the whole path will be created.

You can use it in plugin constructor, or if for some reason configuration extension is resolved asynchronously you may resort to asyncInit() method

class MyPlugin {
  constructor(serverless) {
    this.serverless = serverless

    const value = {
      myKey: 'myValue',
    }
    this.serverless.extendConfiguration(['custom', 'myPlugin'], value)
  }
}

module.exports = MyPlugin

If your plugin needs merging you need to take care of it yourself.

class MyPlugin {
  constructor(serverless) {
    this.serverless = serverless

    const currentConfig = this.serverless.configurationInput.custom.myPlugin
    const value = Object.assign(currentConfig, {
      myKey: 'myValue',
    })
    this.serverless.extendConfiguration(['custom', 'myPlugin'], value)
  }
}

module.exports = MyPlugin