storybook/docs/snippets/common/storybook-main-using-existing-config.js.mdx

```js
// .storybook/main.js

const path = require('path');

// your app's webpack.config.js
const custom = require('../webpack.config.js');

module.exports = {
  webpackFinal: (config) => {
    return { ...config, module: { ...config.module, rules: custom.module.rules } };
  },
};
```