Visa renewal in Thailand

My husband and I have been in China since October 16th/12. We plan to go to Thailand for 26 days at the end of January until the 17th February. Currently we have a visitor's visa with two entries that will expire in April/13, we would like to extend them while in Thailand (even though they're not due to expire until april/13) because we would like to stay in china until the 7th July (and don't plan on another out-of-the-country trip) Can we ask for an extension before they expire and can we do this in Chiang Mai? What paperwork do we need?