Here is what I did. I have only text field for Gender.

"Gender":{
    "type":"text"
}

I want to add a keyword sub field to it. I update mapping as

"Gender": {
  "type": "text",
  "fields": {
    "keyword": {
      "type": "keyword"
    }
  }
}

I use this script to set keyword sub field. Updating the main field automatically updates all its sub-fields.

ctx._source.Gender = ctx._source.Gender.text;