How can I mount a file with the contents of key/value of a ConfigMap without clearing the folder?

I created a deploymentconfig.yml to deploy an app and a want to mount a file which contents are stored in a ConfigMap. When mounting, the files in the folder that is mounted are replaced. The goal though is to only add/remove/overwrite a specific file in that folder.

spec:
    containers:
          volumeMounts:
          - mountPath: /path/toaSubPath/
            name: somename

Is this possible in this deploymentconfig? If so, how can I do that?

Yes, i am using this to mount default configs. Just use subPath and the file name in subPath. Find below sample it works like a charm

spec:
  selector:
     matchLabels:
       name: some_name
  template:
    spec:
      containers:
      - args:
        - bash
        - entrypoint.sh
        image: xyz
        imagePullPolicy: IfNotPresent
        name: some_name
        volumeMounts:
        - mountPath: /full/path/to/be/mounted/default.json
          name: config
          subPath: default.json
      volumes:
      - configMap:
          defaultMode: 420
          name: config