Many tutorials cover this, but I can explain. You don't need to use JASS.
Create a custom ability. You can base it on pretty much any passive ability. Base it on the Capture the Flag item ability if you want to make sure you can find the right fields to change.
Then go to Art - Caster or Art - Target (can't remember, but if you base it on Capture the Flag, you can see which of these that has an effect), and then change it to the model you want to attach. Then go to the field called Art - Target (or Caster?) Attachment Points and set it to head, chest, weapon,right, or whichever attachment point you prefer.
Now you can give the ability to your unit either via triggers, or simply put the ability on an item, so that when picked up, the unit will have a katana or whatever attached to it.