Hello pleXi and welcome on the forum.
In china expension are free, so you only recharge your account and you are all set. But to play on the chinese servers you'll need to create a new chinese battlnet account, some links here http://wow-in-chinese.forum2jeux.com/general-f1/account-management-china-mainland-taiwan-t1492.htm
For the ID you gotta ask one of your chinese friends to let you use his. For legal reasons we don't provide IDs on the forum.
As for recharging the game you can find gamecards in most newpaper shops and convenient stores. Just ask for 魔兽世界卡(moushoushijie ka), they cost 30RMB. You can also buy on taobao if you are familiar with it. Not sure they accept blue cards tho.
Wish you goodluck on setting up your stuff and if you need more help you are welcome to ask.
EDIT: As for the client I suppose you already have a cataclysm one, so follow the sticky guide in the technical forum, you'll have to use a custom launcher to play on the mainland realms.
When you log on a realm type "/join english
" to get in touch with the community members.
WoW English/Chinese glossary
(2000+ translated words) found on wow-in-chinese.jfs-web.chEnglish speakers are consolidating on the same realm for Cataclysm check this thread