#!/bin/bash

input="{
  \"VariaB\": \"DONE\",
  \"VariaC\": \"DONE\",
  \"VariaD\": null,
  \"VariaE\": true
}"

VariaA="ABCD"
VariaF="false"

printf "{
  \"VariaA\": \"$VariaA\",
%s
  \"VariaF\": $VariaF
}" "$(grep -v '[{}]' <<< "$input")"