Here's how to do it:
1. Buy 2 weapons. One, that u want to enchant, and one with the same grade, doesnt matter which one - you will be using the same enchants.. In this example you buy Draconic bow and Shining Bow and you want to enchant Shining.
2. Enchant Shining to your max safe.
3. Then put Shining in the window and start enchanting it above the safe enchant limit. When the progress bar reaches "0", right after glowing animation and before the window says "Succes! You now have xxxx weapon" put Draconic Bow in the enchant window (in the center, like u were enchanting it). The window will say that u have enchanted Draconic bow but the result is its still on +0 and the Shining Bow was enchanted succesfully
(look in inventory). Now i dont know if it works 100% on all gracia servers but i tested this on mine and some HR l2j gracia server and it worked. You just need to time-in well. But beware - if u play on laggy server u can easily brake your weapon if u put the other one too late.Here's the proof:
no tools, no hacking, no warehouse bugs, no trading etc - u just enchant and it works.
VIDEO:
(credits for finding the video: anderpr)
but it still works on lucky 
