Oh, since I don't yet know whether the Mastodon APIs allow me to write an importer which would insert stuff earlier than existing content, here's a link to my old content:
@ssokolow No, you won't be able to change the clock back, and post with earlier time lines. I would suggest you could have a mention and a link to the Twitter profile in your Mastodon profile here, so people can find it easily and follow thru if they like.