It is possible but as i have known and read from past seniors, we would suggest u include yr girlfriend now as as yr common law spouse if u guys are staying together already. This would prevent any "cheating" detected by the immigration officer later after u have applied and changed yr status as married.
Mate the concept of girlfriend (or extra marital relationships) doesnt exist in many cultures so please take that into account when responding to such queries.
Karim, it shouldnt be a problem. You can have your PR processed now and can sponsor your future wife later on from Canada. Also in case you have to be outside Canada to get married and want to take your partner with you, then you can get a visit visa for her through the closest Canadian High Commission and then apply for the PR once she gets there. Hope it helps.