I got JsonMangler working after much trial and error, and uploaded the transformed tiddlers here using the plugin method Data/ATBRecipes.

Various other tips from Joshua in an old thread.

The SQL I used, for 297 results, was this:

SELECT t.title, p.raw as text, p.created_at as created
FROM posts p
LEFT JOIN topics t ON t.id = p.topic_id
WHERE t.archetype != 'private_message'
AND t.user_id = 2
AND p.user_id = 2
AND t.category_id IN (1,21,19,24,18,23)
AND t.deleted_at is null

On import, I was using the title, but there are duplicate titles, so they get overwritten.

Need to post about this on the forum and ask Joshua about appending a row number. Or, I might be able to export with a post id and use that column name?