JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.0.0-beta1/jquery.js"></script>

JavaScript

var obj = {u1fsQExd1aZmnpL : {
    F2NdVdkaefaD6xQ : {
        fZ0zn6d51TgVqID : {
            E051JSFYVHJlGVP : {}
        }
    }
}};

function get_into_subfields(form_structure) {
  for(var mainid in form_structure) {
    if(!form_structure[mainid].hasOwnProperty('subfields')) {
      form_structure[mainid]['subfields'] = {};
    }
    for(var key in form_structure[mainid]) {
      if(key != 'subfields') {
        form_structure[mainid]['subfields'][key] = get_into_subfields(form_structure[mainid][key]);
        delete(form_structure[mainid][key]);
      }
    }
  }
  return form_structure;
}

get_into_subfields(obj);
console.log('obj', obj);