just create another hero and tweak it to be like the backpack, make it invul, have a movement type of none etc, link it to the owning player, then have a function that runs every .xx seconds which puts that bakcpack to the position of the actual hero so that you won't have a problem when transferring items...
if you want the items to automatically go to backpack, then you can either use the event a unit is issued an order then check if it's to pick an item, or make the item drops as tomes and have an event that creates the actual items when you pick up a tome and give the item to the backpack of that hero/player...