If it's preplaced, just do the event:
Unit uses an item
And conditions:
Item-type equal to (Your item)
(Hero manipulating (Item being manipulated)) equal to (Your hero)
And actions:
Replace (Your hero) with a (Your unit) using the units relative hp and mana
If it's not preplaced, set a variable to it. If you want the unit to have the same exact hp and mana, you'll have to do either game caches, or make the unit a hero, with the same intelligence, strength, and Agility. OR, you can adjust the morph ability.