Rocket chat high availability with kubernetes

I am trying to setup HA rocket chat with kubernetes. According to the rocket chat HA guide, rocket supports HA. I assume StatefulSet should be used to support such records:

- "rocket-1:"
- "rocket-2:"
- "rocket-3:"

However i still miss some parts of the puzzle. Where in kubernetes can i specify extra_hosts? Is where any guide of how to achieve it?