How to create nested object when transfering data from one index to another via logstash

Elastic Stack Logstash
1 
The original index has a mapping:
   "GTIN": {    
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        }


And target index has mapping:
 "SAPEPCExtension": { 
          "properties": {
            "EPC": {             
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            }, 
            "GTIN": {          
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            }