Try to do this in order to extend feature this might help you :

<?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\Customer\Api\CustomerRepositoryInterface" type="Vendor\ModuleName\Model\ResourceModel\CustomClassName" />
</config>

Try to do this in order to extend feature this might help you :

<?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\Customer\Api\CustomerRepositoryInterface" type="Vendor\ModuleName\Model\ResourceModel\CustomClassName" />
</config>

Make sure your model code look like this :

<?php
namespace Vendor\ModuleName\Model\ResourceModel;

use Magento\Customer\Model\ResourceModel\CustomerRepository;

Class CustomClassName extends CustomerRepository
{
   ...................
   ...................
   ...................
}