You can import from any blog that has an Atom feed. The number of posts and the amount of content shown in each post will depend on what the Atom feed provides, so if the Atom feed contains excerpts of the 10 most recent posts, then that’s the information that can be imported. We have additional Atom API support for TypePad and Blogger blogs, which is why you need to provide the sign-in information when you’re importing from those services. When importing from TypePad and Blogger, you can choose from your 25 most recent posts.
We are looking into providing Atom API support for other blogging services in the future. Also, we anticipate the ability to import more posts and older posts from your other blogs.
This is great news for people who would like to migrate their blogs to Vox (which I maintain is one of the best personal blogging platforms around).