Copying the:
[x:]\World of Warcraft\_retail_\WTF\Account\[your account]\[character realm]\[charcter]\chat-cache.txt
From one character to another should work.
Make sure you are copying from/to the _retail_ path (the location of WTF has moved under this folder).
Make sure you are completely out of the game when you copy/paste (rename/save the old file as well).