Apache Nifi: Modify JSON Object List

I need a small help in modifying the JSON file by a content of its own object.

My Json array looks like below

[
  {
    "firstName": "John",
    "lastName": "Wick"
  },
 {
    "firstName": "Lisa",
    "lastName": "Tandon"
  }
]

The final output expected is as below

[
  {
    "firstName": "John",
    "lastName": "Wick",
    "fullName": "John Wick"
  },
 {
    "firstName": "Lisa",
    "lastName": "Tandon",
    "fullName": "Lisa Tandon"
  }
]

I have tried with ReplaceText, but not able to access content inside Json.

PS: Won't be able to make Json to attribute and back to AttributeToJson as data is huge considering number of parameters (all not listed above)

Can anyone help me what should be replaceText formula or Jolt formula