Adding custom key and value to bucket aggregation response

Is there a way to add a custom key and value to bucket response like the one below:

{
  "took" : 7685,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 10000,
      "relation" : "gte"
    },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "level_two" : {
      "doc_count" : 308153,
      "bg_count" : 1079172,
      "buckets" : [
        {
          "key" : "Español",
          "doc_count" : 89027,
          "score" : 0.7228597097913709,
          "bg_count" : 89027
        },
        {
          "key" : "Deutsch",
          "doc_count" : 48543,
          "score" : 0.39414760569717644,
          "bg_count" : 48543
        },
        {
          "key" : "Français",
          "doc_count" : 36772,
          "score" : 0.29857231231478437,
          "bg_count" : 36772
        },
        {
          "key" : "Català",
          "doc_count" : 23194,
          "score" : 0.18832498128546465,
          "bg_count" : 23194
        },
        {
          "key" : "Italiano",
          "doc_count" : 23188,
          "score" : 0.18827626394961433,
          "bg_count" : 23188
        }
      ]
    }
  }
}

The desired response:

{
  "took" : 7685,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 10000,
      "relation" : "gte"
    },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "level_two" : {
      "doc_count" : 308153,
      "bg_count" : 1079172,
      "buckets" : [
        {
          "custom_key": "All the same text",
          "key" : "Español",
          "doc_count" : 89027,
          "score" : 0.7228597097913709,
          "bg_count" : 89027
        },
        {
          "custom_key": "All the same text",
          "key" : "Deutsch",
          "doc_count" : 48543,
          "score" : 0.39414760569717644,
          "bg_count" : 48543
        },
        {
          "custom_key": "All the same text",
          "key" : "Français",
          "doc_count" : 36772,
          "score" : 0.29857231231478437,
          "bg_count" : 36772
        },
        {
          "custom_key": "All the same text",
          "key" : "Català",
          "doc_count" : 23194,
          "score" : 0.18832498128546465,
          "bg_count" : 23194
        },
        {
          "custom_key": "All the same text",
          "key" : "Italiano",
          "doc_count" : 23188,
          "score" : 0.18827626394961433,
          "bg_count" : 23188
        }
      ]
    }
  }
}

The custom_key will be the same for every bucket.