AWS DMS: OpenSearch target - nested JSON document

Hi All I need to work on a MS-SQL to AWS Opensearch one time data migration The data is in denormalized form - e.g. we have order table & order lines table But in JSON documents in OpenSearch indexes, we need to combine the denormalized data The order header (first 2 fields in below JSON) and the order lines have to be combined like this -

{
   ordNum: 23,
   custNum: "cust-id12",
   orderlines: 
   [{
       ordLineNum: 1,
       productSKU: "item-id11",
        qty: 2
    },{
       ordLineNum: 2,
       productSKU: "item-id22",
        qty: 3
    }]
}

I found this https://aws.amazon.com/blogs/database/introducing-amazon-opensearch-service-as-a-target-in-aws-database-migration-service/

But there is way no to nest JSON documents in existing JSON. or combine 2 tables with header+details in 1 JSON document (and migrate to Opensearch). Could someone let me know how this can be achieved using AWS DMS

Thanks in advance.