Add a task action to a workflow definition using Power shell

This could be possible using powershell. However, I recommend not to do it as this is an unsupported way and requires quite heave xml manipulation.

Using a powershell script you can either use NWAdmin tool or Nintex Webservice to export a xml workflow file and publish/deploy a workflow using this xml file. Bewteen export and publish you would have to use powershell to manipulate the xml file and add the task action as required. However, afaik there is no public documentation that explains where and how to add actions by editing the xml file. Therefore you would have to use and existing xml workflow file that includes a task action as a blueprint.

Nintex Webservice reference

NWAdmin tool reference

See this example provided by @vadim_tabakman on how to use NWadmin tool from powershell. And this example for using nintex webservice form powershell.

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@Vidya_CK wrote:  I believe that the new task action will take effect with workflows that will be triggered new and not for existing in progress workflows.

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That is correct.

Keep us informed about your progress.