Well too much substrate COULD encourage aestivation therefore I would not do that if I were you; especially since its getting colder and frogs normally aestivate when weather conditions are bad/too dry. Also because it could end up bad if you are not prepared or don't know what to do in that situation. If the problem is making the cage warmer during a colder season, adding more substrate will not help because you will just end up with a larger quantity of cold substrate. You can obtain a lamp with a dimmer switch and a 50w incandescent bulb ( RED/ Infrared preferred so it would not disturb your frog's day cycle) and you can adjust the bulb potency to reach the temperature you desire.

Also following Carlos' advise will really help you sustain temperatures and humidity levels.