I think it will be difficult to create a news feed item via Nintex Workflow. Basically news feed is just another List in SharePoint but it depends on specific guids and architecture to built it's feed. I wouldn't mess around there to not break anything but maybe someone else has better experience here.

Another option might be Yammer though. If you're already using yammer you could add a yammer webpart to your site and use this instead of a classic newsfeed. Nintex workflow work great with Yammer to create entries.

Kind regards

Enrico