title: Mount (add) files to existing directory using configmap volume mount
body: <p>I have a ConfigMap with multiple files, and want to add these files to an already existing directory. But the tricky part here is, the filenames(keys) can change. So I can't try to mount them individually using subPath.</p>
  <p>Is there any way this can be achieved from Deployment manifest?</p>
  <p>Configmap:</p>
  <pre><code>config-files-configmap
  └── newFile1.yml
  └── newFile2.yml
  </code></pre>
  <p>Existing directory after adding files from configmap:</p>
  <pre><code>config/
  └── existingFile1.yml
  └── existingFile2.yml
  └── newFile1.yml
  └── newFile2.yml
  </code></pre>
  <p>PS: I have tried mounting the configmap as directory, which will override existing contents of the directory.</p>
  <p>Thanks</p>