Solution:
You would need to export the posts from Umbraco in xml/json format and then import this into your wordpress site.
I would suggest you either check if there’s already RSS features in your blog that would allow you to read all the posts as XML – if that is not the case, it should be fairly easy to create a new template in Umbraco, simply looping through all descendant nodes in the blog, showing the content of each blog post as XML data instead of rendering HTML for the browser.
Then you would need to import this in WordPress somehow. The WordPress part I’m not that familiar with but there’s most likely some plugin that can do this for you.
I don’t know of any packages/plugins that can help you do exports (only imports) from umbraco since the exports are usually easier to do manually so the format is what you expect.