I guess it could be good to see a vet who might be able to rule out any infection or prescribe something, but this thread might help on the diet - it's actually for kidney failure but there are some general tips and dietary advice that might help as well.
Chronic Kidney Disease / Chronic Renal Failure: diagnosis, "treatment" options
I know herbs can't be given willy nilly, but if you did see the vet you could maybe ask if something like that would help. Rodipet actually make a "porridge" for liver, bladder and kidney that has milk thistle, nettle, dandelion root, willow bark and goldenrod in. I noticed the post above suggested milk thistle.
Although maybe it isn't appropriate for an older hamster, but they are meant to improve function, not just be diuretics and given in the right dose.
https://www.rodipet.de/shop/futter/z...ase-niere.html