No! You can start from scratch, or from any content (that you own!) that has an RSS feed. Any content with an RSS feed can be imported into Leanpub. This includes different blogging platforms (LiveJournal, Blogger, Typepad, etc) and microblogging platforms (Twitter, etc). Or, you can start from scratch and write the entire book on Leanpub. If you import your RSS feed, what happens is that each RSS feed entry (blog post) turns into a file in your Dropbox folder.