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