This might seem like a duplicate, but other questions I've found seem to be about exporting/importing Wordpress, as a 1 shot deal.
My situation here is quite different; I'm looking for a method/plugin that would write in the posts as they are posted on another site.
Here is a more detailed way to explain it. My client has a blog, with his new site. He also has 3 kind of subblogs on other sites on other domains. The behavior we want is that if he posts something on the subblogs, it gets also posted on the main, new blog.
We can't run on multisite, the subblogs are already been running for a year, and there's enough traffic on it to not being able to take them down to make an update.
Anybody would know a plugin, or a method within functions.php (We do have access to the code) that could make that happen?