Hi Philippe,

I think a solution would be to use a Joomla! module :

1/ Create a module with "Seblod - Form" type

2/ Call Category form into

3/ Create a custom position (for that, write the name you want (ex : "addcategory") and create this position into the category field)

4/ Go to your article content type and create a Joomla! module field type

5/ Into the field configuration (click on the pen) write your position name and it would works !

If you have other questions or if it doesn't work don't hesitate to come back to us (with screenshots).

Good continuation,

Ness

Correct me if I'm wrong, but I don't think form such form in form would work. To create a record you would need a Free button opening separate category form with redirect back to the article form after category is saved.

To have a check whether category exists, you would need to use code beforeStore field and using some php check if category exists and if it does stop execution.