Services do not have a volumes from option in the UI because it doesn’t make sense… The service can have many containers (scale option) on different hosts.
We call what you’re looking for Sidekicks, http://docs.rancher.com/rancher/concepts/#sidekicks . This lets you define a sub-service that gets scheduled alongside and at the same scale as the primary service. Creating sidekicks is currently only in the rancher-compose CLI, not the UI.