Here are examples of different methods for appending values to arrays and lists in automation scripting.

Using var.push:

start:
  set:
    emails@csv: boss@cerb.example, customer@cerb.example, accounting@cerb.example
  var.push:
    inputs:
      key: emails
      value: marketing@cerb.example

Using var.set:

start:
  set:
    emails@csv: boss@cerb.example, customer@cerb.example, accounting@cerb.example
  var.set:
    inputs:
      key: emails:{{emails|length}}
      value: marketing@cerb.example

Using |merge:

start:
  set:
    emails@csv: boss@cerb.example, customer@cerb.example, accounting@cerb.example
  set/append:
    emails@json: {{emails|merge(['marketing@cerb.example'])|json_encode}}

Using |csv and string concatenation:

start:
  set:
    emails@csv: boss@cerb.example, customer@cerb.example, accounting@cerb.example
  set/append:
    emails@csv: {{emails|join(',')}}, marketing@cerb.example