Mocking runtime config value with sinon

I am adding some config values before hapi server start. Application is works fine although in test I can not use config.get(). I can get around with proxyquire. So I was wondering

• Is adding config file "dynamically" is bad design?
• Is there a way I can use config.get() in such suitation?

• Any alternative approach?

  //initialize.js
  
  const config = require('config');
  
  async function startServe() {
    const someConfigVal = await callAPIToGetSomeJSONObject();
    config.dynamicValue = someConfigVal;
    server.start(); 
  }
  
  //doSomething.js
  
  const config = require('config');
  
  function doesWork() {
    const valFromConfig = config.dynamicValue.X;
    // In test I can use proxiquire by creating config object
    ...
  }
  
  function doesNotWork() {
    const valFromConfig = config.get('dynamicValue.X'); 
    // Does not work with sinon mocking as this value does not exist in config when test run.
    // sinon.stub(config, 'get').withArgs('dynamicValue.X').returns(someVal);
    .....
  }