Hi,
There is nothing wrong with supplementing their natural diet with things like meat pet food, plain minced meat or cat biscuits (or even proper chog biscuits!) and during bad unseasonable weather they will surely be grateful, A few bowls of drinking water around the garden will also be received well but be sure not to do it too often as they may come to rely on you for food. Feeding them bread as someone on here said is a definite no no as is giving them plain milk as it can cause diarrhoea which is a serious condition for them, especially when food/water is short. If you wanted to give them milk, then goats milk or at a stretch watered down cows milk (at least 50/50) could be beneficial before they hibernate.
It's good to give nature a helping hand but as you have already hinted at, just not too often as to make them dependant.