customPropToClassName.js

// customPropToClassName.js
const traverse = element => {
  if (element.props) {
    let props = {};

    if (element.props.customProp) {
      // Inject className or anything you want
      props.className = 'important-class';
    }

    if (element.props.children) {
      props.children = Array.isArray(element.props.children) ?
        element.props.children.map(child => traverse(child)) :
        traverse(element.props.children);
    }

    return React.cloneElement(element, props);
  }
  else {
    return element;
  }
};

// Secret sauce: We extend the decorated component and override its render method.
export default WrappedComponent => class extends WrappedComponent {
  render(...args) {
    return traverse(super.render(...args));
  }
};

//-------------------------

// App.js
import customPropToClassName from './customPropToClassName';

@customPropToClassName
export default class Example extends React.Component {
  render() {
    return (
      <div customProp='something' id='wrapper'>
        <p customProp='another'/>
      </div>
    );
  }
}