How can i make my syrian hamster gain weight?

i have a gray syrian hamster named Roxi, she is about a year old, she is too large for her weel so she runs around in her exercise ball alot and in a large bath tub, i dont want to limit her time out of the cage so do you have any ideas on what i could feed her to get her to a more healthy weight? i can just about see her ribs and im worried she might be underweight. she is active and playfull but maybe i could try help her gain that little bit extra to make her more healthy? she eats a high quality hamster mixed food and i give her treats of cheerios and apple slices oacasionally, do u know of any treats i could give her to help her gain weight?