Well, there are a couple of ways to do this but they will all require you to create a new version of every ability that the item will effect for every time this item's effect will stack.
If you didn't need the cooldown or mana to decrease, just a greater effect, you could trigger the effects of all the spells and use conditions to check if the unit has the item and, if so, upgrade the damage/speed boost ect. in the trigger.
The only problem is that you couldn't use more than one of these items at a time.