What kind of hub motor is it ? It probably would be able to handle it unless part of your regular ride is a sustained uphill section that lasts 1/2 mile or something, then it could possibly overheat the motor. Most every motor is capable of overheating. The crystalyte X5's are the best at being able to handle lots of volts without overheating.
If you don't ride in rain or mud, some people have drilled symmetrical holes in their motors to allow for better heat dissipation. I've heard that drilling holes in the wrong places can make your hub motor out of balance and start to wobble too so be careful.
What kind of speed/acceleration are you looking for? It might be better to upgrade the motor and controller rather than buy more batteries. Look at both options and see. My 5303 goes about 30 mph at 36v so sometimes you can get more out of a different motor with less voltage. Most motors only go about 23 mph at 36v.