Use dart json_serializable to map an object field to JSON flat keys (one to many mapping)

I have a dart object that includes a field of type `Money`, which itself is composed of `amount` and `currency`:

```
@JsonSerializable()
class Account {

final String id;
final String type;
final String subtype;
final String origin;
final String name;
final String status;
final String currency;
final Money balance; <== value object
...
}
```

`Money` looks something like this:

```
class Money {
final int amount;
final String currency;

const Money(this.amount, this.currency);
...
}
```

The above is to be mapped for use by `sqflite`, therefore the target JSON must be a flat JSON, like:

```
{
"id": String,
"type": String,
"subtype": String,
"origin": String,
"name": String,
"status": String,
"currency": String,
"balanceAmount": int; <== value object
"balanceCurrency": String; <== value object
}
```

I understand I can use `JsonKey.readValue` to extract a composite object from the entire JSON object prior to decoding.

**But how can I do the opposite? Grab the two values from the `Money` instance and map them to `balanceAmount` and `balanceCurrency`?**

Nothing in the search I did on `json_serializable` api docs, GitHub issues, or StackOverflow appears to respond to this in particular: how to map one field to two (or more) target keys?