Yes, I found it : model/modx/import/modimport.class.php
And I also found a solution to import my data without modification.
I changed the structure of the files I want to import with:
<html>
<head>
<title>My Title</title>
</head>
<body>
<date>2010-03-10</date>
<content>
<p>The content</p>
</content>
</body>
</html>
and in the "Enter the root HTML element to import:" field I put
{"content":"$content","pub_date":"$date","published":"1","longtitle":"$title"}
In french translation the help text is a bit misleading.
In english it says "Provide JSON with associations "field":"value". If value starts with $ it is jQuery-like selector.
Field can be a Resource field or TV name."
In french : "Fournit un ensemble JSON avec des associations « champ »:« valeur ». Si la valeur commence par $ c'est un sélecteur de type jQuery.
Le champ peut être une ressource ou le nom d'une TV."
If I translate the last part of the french text that would be : "Field can be a Resource or TV name" (without the "field" word). I think the french translation should be : "Le champ peut être un champ de ressource ou le nom d'une TV."
[ed. note: JuTs last edited this post 10 years, 4 months ago.]