Yes there is no problem.
Used this annotations in my dummy nginx pod:
annotations:
co.elastic.logs/processors.decode_json_fields.fields: 'message'
co.elastic.logs/processors.decode_json_fields.target: "qrapp"
co.elastic.logs/processors.decode_json_fields.add_error_key: 'true'
And see the result:
Mind that this is the json entry I create in my message field:
'{"test":"hello"}'

