Is it possible If I request 30 days of stay in China for an extension visa ?

You can apply to the local Exit and Entry Administration once you are physically in China. However, the officials have total discretion over your application, and if the extension is denied you must leave China before your original duration of stay expires. Otherwise there will be consequences such as fines or police detentions, any of which will render you ineligible to use visa-free transit and may potentially cause your subsequent visa applications to be refused.