Copy single field from one index to another existing index

Is it possible to take a field from one index and copy it to another existing index without loosing fields it already contains?

For example I have an index abc having fields field_a, field_b, field_c and index def having fields field_d, field_e, field_f. What I want to achieve is to have index def having fields: field_d, field_e, field_f (already existing in def) and field_a (taken from abc). Both indices are identified by the same _id but documents have different types.

I was trying to use _reindex API:

    POST _reindex
    {
        "source": {
            "index": "abc",
            "_source": [
                "field_a"
            ],
            "query": {"match_all": {}}
        },
        "dest": {
            "index": "def"
        }
    }

but it replaces currently existing documents in def instead of just adding a field.

Currently, I'm using Elasticsearch 5.6.

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