It is possible to extend an existing (http://blog.abhinav.com/extending-your-%E2%80%9Ctemporary-stay%E2%80%9D-in-canada/) work permit in Canada!
The application meant for the extension purposes are required to be submitted to a Case Processing Centre located within Canada and it is essential that the concerned applicant must not leave the premises during the commencement of the application process.
It would be a wise decision to apply before two months before the expiry date in order to be on the safer side. In addition, it is essential that the employer must get an affirmation from Human Resource Development Skilled /Service Canada in order to renew the work permit. After a positive outcome, the worker can apply for the extension.
But in some cases, work permit are not extendable. For instance, in certain cases of “post graduate work permit,” employers are not required to get confirmation from Human Resource Development Skilled Canada/Service Canada. But if the employers intends to continue with the same person at the end of the work permit tenure, then it is mandatory that he goes through the confirmation application process, but the chances of success are NOT GUARANTEED! Such cases require an expert guidance from an Immigration and Visa consultant. So hire one today!