Using Kettle version 4.4.0

I am migrating a SQL Server 2008 DB to MongoDB

Everything is working great except one problem..

How to have row values as KEYS in MongoDB Key-Value pair. Right now in the MongoDB Output step, I can define the path, where the KEYS are hardcoded. What if I want to the values returned from SQL Server Table Input as Keys.

E.g.

This is normal JSON structure with Hardcoded Keys..

{ "top1" : { "first" : "<string val>" }, "array" : [ { "last" : "<string val>" , "address" : "<string val>"}], "age" : "<integer val>"}

What is instead of this I have a following JSON..

{ "top1" : { "#{the_value_contained_in_column_name}" : "<string val>" }, "array" : [ { "last" : "<string val>" , "address" : "<string val>"}], "age" : "<integer val>"}

Simply saying, the keys need to be dynamic.. If this is not possible any workarounds are also welcome..
Thanks in advance