Hi, I'm experiencing exactly the same problem right now. Has a solution been found in the meantime?
My goal is to add an additional textarea field to the category form in the backoffice to create a second category description.
Since my PrestaShop version is 1.7.6.5, it seems not to be able anymore to override the controllers/admin/AdminCategoriesController.php class.
I also found the stackoverflow question linked above, but it seemed to me to be kind of incomplete, because on the one hand probably the new field has to be created in the database and on the other hand the hooks "actionAdminCategoriesControllerSaveAfter" and "actionAdminCategoriesFormModifier" do not seem to exist.