You need a custom plugin and/or theme to add new fields to categories.
Hmm you should be able to display some content only after the first posts by adding a if in the topic.tpl. For example on this line https://github.com/NodeBB/nodebb-theme-persona/blob/master/templates/topic.tpl#L74
You can add
Since the main posts has an index of 0, that content will only show in the main post.
I can see the endpoint to create a single topic. Is there an endpoint for more than one topic creation in a single call? i.e by passing an array of topics.