Adding Metadata to JSON Payloads

I need to provide a JSON feed that looks like this:
{"redirect_flows": {
    "description": "Wine boxes",
    "session_token": "SESS_wSs0uGYMISxzqOBq",
    "success_redirect_url": "https://example.com/pay/confirm", "prefilled_customers": { "email":"emailaddress","given_name":"firstname","family_name":"lastname","address_line1":"address1 Road","address_line2":null,"city":"London","postal_code":"postcode","country_code":"GB" },  "links": { "creditor": "CR123" }}}

I've been using the JavaScriptSerializer to serialize the data   in a similar BUT simpler JSON feed and using replace to tweak the feed to go from this:
[{"email":"emailaddress","given_name":"firstname","family_name":"lastname","address_line1":"address1 Road","address_line2":null,"city":"London","postal_code":"postcode","country_code":"GB")]
to this:
{ "customers" : { "email":"emailaddress","given_name":"firstname","family_name":"lastname","address_line1":"address1 Road","address_line2":null,"city":"London","postal_code":"postcode","country_code":"GB", "metadata": {"customerid":"124"}}}

Can someone help me and show me how I can add the metadata  programmatically and more importantly scalable so I don't have to use the replace etc. every time?

Thanks

Mark