How can I Add/run WindowsupdateDiagnostic tool on any target machine from pdq inventory

Because you can't specify a computer parameter for Get-TroubleshootingPack and because you have to first create an answer file and then point to that answer file in order run the tool silently i would suggest running this as a package as apposed to an Inventory Tool. Follow the steps below.

Step 1:

Create an answer file called "WUDAnswers.xml" using the following lines.

For Windows 8/8.1 and 10

<?xml version="1.0" encoding="UTF-8"?>
<Answers Version="1.0">
<Interaction ID="INT_AggressiveMode">
<Value>1</Value>
</Interaction>
</Answers>

For Windows 7

<?xml version="1.0" encoding="UTF-8"?>
<Answers Version="1.0" />

Step 2:

Create an package with a PowerShell step containing the following.

Get-TroubleshootingPack -Path $env:SystemRoot\diagnostics\system\WindowsUpdate | Invoke-TroubleshootingPack -AnswerFile .\WUDAnswers.xml -Unattended -Result $env:HomeDrive\WUDResult

Step 3:

Attach the "WUDAnswers.xml" file you made earlier to the files field of the same PowerShell step you just created.

The results of the scan should be located inside "%HOMEDRIVE%\WUDResult" (C:\WUDResult). The results aren't easy to read, but the scan does seem to work.

You could also download and extract the "windowsupdatediagnostic" folder from the "WindowsUpdateDiagnostic.diagcab" file and point the powershell script to that, you may need to create your own answerfile using the following script.

Get-TroubleshootingPack path\windowsupdatediagnostic -AnswerFile $env:HomeDrive\WUDAnswers.xml