Welcome to the Fediverse! Get the fuck off mastodon.social as soon as possible
What frustrates me is that it's entirely possible to write an external app to do post-migration. it wouldn't have to run on the same server or even be in the same language (Masto is written in Ruby). I just haven't had time...
(The one catch is that I'm not sure if the API allows backdating new posts -- but that could be ameliorated by adding a header or footer indicating that the post was migrated, and its original timestamp.)