Post graduate work permit is not extendable. The only other way is to get your employer to apply for LMO and then apply for a close work permit ( you can only work for sponsored employer)
Or for bridging permit after you have apply for PR
1) PNP.(you need employer's letter, rest of Rules for PNP differ based on Province )
2) LMO(you need employer's letter)
3) CEC. (you got File number for your PR application)