It's possible to create a pipelinerun with non-existing field serviceAccountNames

---
apiVersion: v1
kind: ServiceAccount
metadata:
  name: sa-for-task2
---
apiVersion: tekton.dev/v1beta1
kind: Pipeline
metadata:
  name: hello-pipeline
spec:
  tasks:
  - name: task1
    taskSpec:
      steps:
      - name: hello
        image: registry.access.redhat.com/ubi8/ubi-minimal
        script: |
          #!/usr/bin/env bash
          echo "Hello from task 1"
  - name: task2
    taskSpec:
      steps:
      - name: hello
        image: registry.access.redhat.com/ubi8/ubi-minimal
        script: |
          #!/usr/bin/env bash
          echo "Hello from task 2"
---
apiVersion: tekton.dev/v1beta1
kind: PipelineRun
metadata:
  generateName: hello-pipeline-
spec:
  pipelineRef:
    name: hello-pipeline
  serviceAccountName: pipeline
  serviceAccountNames:
    - taskName: task2
      serviceAccountName: sa-for-task2
  timeout: 2m

$ tkn pr ls
NAME                   STARTED       DURATION   STATUS
hello-pipeline-2xqv2   ---           ---        ---