While you are configuring your box, you will notice that below the description field there are several advanced areas. One of the most important is "Fetch posts from an external feed". With this option you can make Blogoosfero copy posts from and external blog.

When you choose this option, the system will open new fields where you will type (or paste) the adress of the feed you want to clone and two other options. If you are migrating your blog to Blogoosfero and leaving the old blog behing, you want  "Fetch posts only once". If you intend to continue updating your existing blog and have your new posts to your copied to your Blogoosfero blog, choose "Fetch posts periodically.