Undead Weapon

Is it possible to make an item that adds attack when attacking undead? And how?

Of course there's a way! There's always a way, only your solution MIGHT require triggers, unless there's an ability i forgot about. Anyways, make a new trigger:
Event:
A unit is attacked

Condition:
Boolean Comparison: Attacked Unit is Undead equal to True *Undead is a classification type, like mechanical"
Boolean Comparison: Attacking Unit has Item of type (Your Item) *This is under Hero, not Unit*

Action:
Here's the tricky part---There's a few ways to go around this. For 1, you could use Lord Vexorian's Caster system to do the damage, *IF* you want it to grant experience and bounty for kills. Another -tricky- way though, if you want to save time, is this.
Make a new ability based off an item ability that adds damage. Change the name to whatever, and make the added damage whatever you want the weapon to add. Now the event "A unit is attacked" does not take place when the unit is hit, it takes place just before then, when the unit is ready to swing at the bitch. So after the ability is done. Put this in the trigger:
Unit: Add ability (The ability you just made) to Attacking Unit
Wait 1 second (just so the unit is done attacking, or even less time maybe)
Unit: Remove ability (The ability you just made) from Attacking Unit

That's it, I hope I helped you. :D

You could also use an Orb of Slow ability that casts Holy Light and give it to the unit as a unit ability. Just gotta make sure it can't target friends, though.