Hello Lionel,
if you look after my precedent post with screen capture, you'll see that it is exactly what I did (ok, it's difficult to see because the image is reduced by forum, don't ask me why).
But when creating a field type categorie, there is a button "multiple". Values are stocked in my custom fied when multiple is off, and aren't where multiple is on.
In fact, the Jeka solution work, that's right, but I like to understand why something is not working ;).
Regards
ps : and categplus field is a varchar(255), not a numeric field, of course.