There are REST APIs provided to Create/Update/Delete/Get a WFA user in WFA. They are available at /rest/docs under UserResource. They can only be called by an Admin user.
If you want a WFA workflow that creates a new WFA User, you would need the following:
1. Create a new command which can call this Rest API. Powershell 3.0 provides Invoke-RestMethod cmdlet which can very easily do it.
2. For creating/Updating a user you need call Invoke-RestMethod, method post/put for creating/updating a user and providing the Admin credentials for -Credentials parameter and an xml file with user details content. And you are done.
3. If you don't have powershell3.0, you'll need to create an HTTPWebRequest, manually add credentials, create a cookie container object and add it then call the API. Long process.
If you want a sample command for Powershell3.0, I'll give you one soon. (somehow unable to find where I saved it )
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