Don`t bother. It could make the drinking water taste funny and stop your hamster from drinking causing dehydration. The best way to supplement a dry diet is to use fresh RAW veg.
Feeding small amounts daily to your hamster with a variety of vegetables is a far better way to administer health benefits. Broccoli eaten raw has been proven to keep cancer at bay.
Is your hamster a Syrian or a dwarf species. Regardless, they all benefit from being fed fresh foods. Harry Hamster is okay but most hamsters like a variety of dry food so you can combine two dry mixes together if they are different ingredients and not too similar. If you have a dwarf hamster for example and are using Harry Hamster, you could buy a bag of Burgess Dwarf food which contains millet seeds which smaller hamsters love.
Supermarkets all sell bags of mixed salad and small mounts of other veg that can be stored in the fridge for a week and replaced when they go past their lifespan. Frozen veg is also fine, but MUST be thawed out in cold water an hour or so before using to room temperature.
Broccoli, cauliflower, garden peas, Lettuce, cabbage, green bean, carrot, cucumber, sweet corn (baby corns or nibblets)....
Just some of the ones hamsters enjoy. Easy to get and keep in the fridge and cut off small fingernail sized pieces and rotate daily. See what your hamster enjoys and only feed in small amounts if he/she has never had any before. They usually tolerate fresh food well but don`t feed too much, too soon. Space it out. Let us know how you get on. x
P.S. Don`t feed grapes to hamsters as there are reports on the RSPCA website to avoid grapes. The alternatives given above are better. x