if I understand you correctly you want to name the field in your cluster <JSON>field2. That is you start with a cluster on your block diagram:
{field1:dbl=1.0,<JSON>field2:string="{"item1":"ss", "item2":"dd"}"}
When you call flatten to json you get:
" {"field1": 1,"field2": { "item1":"ss", "item2":"dd"}} "
because the library automatically pulls off the <JSON> prefix and interprets that whole string as JSON.
When you unflatten the reverse happens,