That is not that easy, because Mastodon sadly does not parse old posts (which they could using the WordPress outbox). There is a way, but it is a lot of handwork: If there are posts you would love to see on the fediverse, you could simply paste the URL into the search box. Otherwise you could edit the post you would like to see.
Maybe I can implement some kind of history feature where the plugin posts some of the old ones, but 1,1k is really a lot!