This is how I did it, there might be quicker and more efficient solutions but this is my way.
(I could have demonstrated via. video or pictures but sadly I do not have the capability to do that)
Weapon:
Copy the entire line of the desired weapon, change the ID to something not taken by other weapons, change the type="look" to "weapon", change the slot="look_rweapon" to slot="rweapon" and, sub_slot="look_lweapon" to sub_slot="lweapon", copy the damage effects and change it upon your desires.
Armor:
Copy the entire line of the desired armor piece, change the id to something not taken by other armor pieces, change the type="look" to "armor", and change the slot="look_?????" to slot="?????" and copy armor settings and change it to your desires.
When done, replace the xitem.xml in the client and the server which has the xitem.xml
NOTE: If you have not inserted the new ID, name etc. in the lang (eng.) xitem.xml you will not find it on the showgmt, to aquire the item @giveitem "id" (without "")
and
It does not matter if you're copying from NA/KOR/JP or any other client, as long as the steps are properly folllowed + the lines has no errors in it, it'll work.
Any more questions send a pm to me.