The only way to extend your post graduate work permit is if you were given a permit when your passport was about to expire. Your post graduate work permit will be about the same time as your passport. You will need to get an extension when you get a new passport. Note that if you need a temporary resident visa to enter Canada, you may also need to extend it when getting other work permits.
There are other ways to extend your stay in Canada. One of them is obtaining a regular Canada work permit. You can apply for either a LMIA or LMIA exempt work permit, depending on for which you are eligible. Another route is by obtaining permanent resident status; you may be eligible for Canada Experience Class due to your valuable Canadian work experience. Alternatively, you still have the following programs:
Federal Skilled Worker Program
Provincial Nominee Programs
Federal Skilled Trades Program
Quebec Skilled Worker Program