While the team at Ghost works to allow users to migrate from other platforms like Wordpress and Tumblr, you can use this tool to create a JSON file which you can import directly to Ghost with your all Tumblr posts.
This tool takes advantage of the exact format of the exported GhostData.json via the Ugly Debug Tools. It maps your Tumblr blog posts data to this format, creating a file that you can import directly to Ghost. It also creates all your used tags from Tumblr so you can use them later in Ghost. Check out an example export.
Hacked together by José Padilla