If you want to add an attachment when picking up an item, you don't need a trigger.
Just modify a passive ability, I usually take Item Damage Bonus
Copy/paste the ability and set the stats to this:
Art - Target: model of the attachment
Art - Target Attachment Point 1: weapon (in case you want dual-wields, you can use attach point 1 for hand, left and the 2nd attach point for hand,right).
Data - Attack Bonus: 0
Set the name to something you can remember
Then go to the item that gives this effect when picked up and add the ability to that item.
It should show the model when picked up.
Note: some units do not have the attachment point "weapon", in that case try "hand, left" or "hand,right".
Edit: Argh, too late...
but not if the unit already has the item
So the unit can wield multiple weapons?
That would suck, since you get multiple sword models through eachother and nothing makes sense anymore.
I advise you to let each unit carry only 1 weapon (or dual-wields, for which you can use hand,left and hand,right).