Kubernetes add conf parameters end of line to the conf file

You can either use the command directive to override the entrypoint of the container, or mount in the conf as a configmap, then edit that.

Here's an example of overriding the entrypoint -- more reading available here

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp-deployment
  labels:
    app: myapp
spec:
  replicas: 3
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
      - name: myapp
        image: myapp:latest
        command: ['/bin/bash -c "echo newvkey=newval >/etc/myconf.conf && ./entrypoint.sh"']
        ports:
        - containerPort: 80