PowerShell how to add something on parsed JSON?

If you’re using PowerShell 3.0/4.0 you can simplify your conversion using the ConvertFrom-Json cmdlet.

Beyond that, if you have PS or .Net Object Types, the Add-Member cmdlet allows you to add arbitrary properties. The following shows how to add a property based on a Json block:

$json = @"
{
  "BlockA": {
    "BlockB": {
      "name": "BlockB",
      "value": "Value_B"
    }
  }
}
"@

$blockcvalue =@"
    {
    "name":"BlockC",
    "value":"ValueC"
    }
"@

$jobj = ConvertFrom-Json -InputObject $json

$jobj.BlockA | add-member -Name "BlockC" -value (Convertfrom-Json $blockcvalue) -MemberType NoteProperty

write-host (ConvertTo-Json $jobj)