You can definitely add, remove, or change all of the fixed properties on the unique items. I don't believe DoP had any skills you could add to the items.
You can let a mage use a shield just by adding
to the mage class.
You might be able to make a wand class that only a mage can use. Instead of using something like a WeaponAxe ItemRequirement you could try something like ItemRequirement Mana instead.
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, Din's Legacy, & Drox Operative 2