Here is a solution to convert comma separated values (like from an iForm MultiSelect Field) into JSON.

In your transformation bucket, copy/paste the following JavaScript in the Transform Code text area

function MultiSelectToJSON(r,multi_list_keys) {
   var keys = Object.keys(r);
 
   for(var i = 0; i < keys.length; i++) {
      if(multi_list_keys.hasOwnProperty(keys[i])) {
         r[keys[i]] = ListToJSON(multi_list_keys[keys[i]],record[keys[i]]);
      }
   }
   return(r);
}

function ListToJSON(key_name,list_of_items) {
   var listAsArray = list_of_items.split(",");
   var listAsJSON = [];
   for(var i = 0; i < listAsArray.length; i++) {
      tmp = {[key_name]: listAsArray[i]};
      listAsJSON.push(tmp)
   }
   return listAsJSON;
}

Then, in the Transform Return text field type the following:

MultiSelectToJSON(record,{"ORIGINAL_KEY":"SUB_KEY"})

Make these two replacements to the above code:

• Replace ORIGINAL_KEY with the Data Column Name of the MultiSelect
• Replace SUB_KEY with the key name you want associated for each value 

If my record has more than one MultiSelect field, separate ORIGINAL_KEY:SUB_KEY pairs from one another with a comma.  For example:

MultiSelectToJSON(record,{"languages":"language","goals":"goal"})

This is before/after snapshots of the record:

Before

{
 "first_name": "Jet",
 "last_name": "Li",
 "languages": "spanish,french,german,korean",
 "goals": "concentration,physical_fitness,meeting_new_people,role_models"
}

After

[
    {
        "first_name": "Jet",
        "last_name": "Li",
        "languages": [
            {
                "language": "spanish"
            },
            {
                "language": "french"
            },
            {
                "language": "german"
            },
            {
                "language": "korean"
            }
        ],
        "goals": [
            {
                "goal": "concentration"
            },
            {
                "goal": "physical_fitness"
            },
            {
                "goal": "meeting_new_people"
            },
            {
                "goal": "role_models"
            }
        ]
    }
]

Hope this helps!