I would like to create or buy a script that would be able to post a new article on Joomla from a WordPress website. I often find the opposite as a solution...
How can code this outside of the local Joomla environment?
You can use the API for Joomla to add articles.
Google is your friend: Using the Joomla! 3.3 API to add articles programmatically
I think obGrabber is the most suitable Joomla extension with what you want.