Overriding or Extending Core Interfaces

Question

I am trying to add a new getter and setter to \Magento\Catalog\Api\Data\CategoryTreeInterface.

I have created a new interface - \Wildcard\CategoriesApi\Api\Data\CategoryTreeInterfaceand a new model - \Wildcard\CategoriesApi\Model\Category.

When I call the /V1/categories endpoint the property is not shown.

However, if I directly add the getter and setter to the core interface, it works.

My di.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">

    <preference for="Magento\Catalog\Api\Data\CategoryTreeInterface" type="Wildcard\CategoriesApi\Api\Data\CategoryTreeInterface"/>
    <preference for="Magento\Catalog\Model\Category" type="Wildcard\CategoriesApi\Model\Category"/>
    <preference for="Wildcard\CategoriesApi\Api\Data\CategoryTreeInterface" type="Wildcard\CategoriesApi\Model\Category"/>

</config>

PHPStorm does highligh the Wildcard\CategoriesApi\Api\Data\CategoryTreeInterface in the type parameter in the first preference as not being able to resolve. I have triple checked my namespacing and file/folder names but they are all correct.

Can someone please help with how to add my methods to the interface?

Using Magento 2.1.7