Pretty clear, thanks! You problem is that you have external data format which you want transfer to. In ideal world I would give this job to an external module that uses API.
If you want to do it entirely inside document, there is the way. Your data structure is very similar to what non-relational databases do, e.g. MongoDB. Each record there has an unique indentifier _id, and the name of this field can’t be changed, but all other fields can: In this case field names are also part of data, not structure.