Might as well plug my homebrewed solution for this: https://github.com/steadmon/blog-replay

Basically, it can scrape a WordPress or Blogger blog into a local DB and then replay the posts one-by-one into an RSS feed.

It's rough around the edges as it was a learning project and I haven't had much time to devote to cleaning it up. But I've been using it for a while and am happy with it.