Keep in mind, you have a TRV that is valid for 6 months. CBSA at the point of entry will determine how long you are allowed to stay, and it may not be 6 months. Think of your 6 month single entry TRV as a test to see if you will abide by the conditions it was approved. I would suspect, if you applied for an extension it would be refused and future TRV’s would become difficult to obtain.