GUI with extra armor slots

I'm developing an item that removes the cooldown of minecraft 1.12.2 items, (so it looks like minecraft 1.8). But for this I need to create an item that increases the overall atackspeed, but I can not make this item work only by being in inventory, it only works if it's like armor. So as I intend to use it in a modpack, I do not want to create an armor specifically for this, but rather make an item that stands apart in an extra slot of armor, so I can use other armor in addition to this item, using the atackspeed increased, because I do not like the combat system with delay of version 1.12.2, so I thought of something to remove it. But since the add-on slots in the MCreator GUI only support common items, I can not turn the item slots into armor slots Please, I really need help, I do not know what else to do! ;-;