Do you have a photo of Flurry. Syrian hamsters can be large or small and she could be right weight for her size, although it does sound a bit low. I'm guessing you feel she does need fattening up a bit
They need a lot of protein in the first year, and tend to be quite lean as protein gets used up and isn't stored like fat, but as Amityville says -if her spine is protruding she could be too thin.
I sometimes supplement with the odd piece of cheese (just a small piece about 1cm cube size) a couple of times a week, which is protein and also fairly high calorie. Sugary treats would help increase weight, but sugar isn't really good for their teeth - they can get dental decay just like humans! Occasionally I will give Charlie one of the Vitakraft honey and seed chew sticks - he goes straight for anything sweet like that - but normally give him sugar free ones. And occasionally he gets the odd hammy chocolate drop (not human chocolate as that can be poisonous to hamsters - hammy chocolate drops are not real chocolate).
What diet does he have? Generally, as long as you put a tablespoon of a good hamster mix out every day or two, they should eat what they need. Supplementing with a bit of fresh veg and healthy treats or the odd piece of plain cooked chicken can be good, but they should get what they need from the hamster mix if that is all he has.
Being underweight could be a sign that there is some illness, so if he really is underweight then a vet check could be a good idea. I'm guessing you just weighed him and thought it sounded a bit low - or does he look underweight?