I figured this one out a long time ago on isogames but it should stil work.
What you need: item, scroll, a 2nd player to help, itemserver lag.
This works on some servers, others might have it fixed.
Here's what you do:
1) Wait for itemserver to lag by spamming antidotes or something.
2) Once the itemserver lags you quickly put a scroll on your item you want to overenchant. Now, if it says 'FAIL', you ask your friend to quickly trade you and send him the item. (you have to do this quickly before the lag clears up)
3) Once the lag is gone, your scroll wil be gone but your item wil be in your friends inventory. (it wil stil be at the same enchant lvl)
I haven't tested this on extreme yet, but they have itemlag every now and then so you might wanna give it a try. Beside, there's way too many people running around with +25 weapons, so I'm pretty sure it stil works there.
Note: Your inventory might get bugged after doing this and you wont be able to trade/use items depending on some unknown factors I'm stil trying to figure out.