We crumble a dry one into the mix as an addition.
You can give them a soaked one but cows milk is not ideal as some hammies cannot tolerate it and it upsets their tummy.
I'd just soak it in hot water then let it cool and give about a quarter of a weetabix biscuit. Leave it overnight and remove what he has left in the morning (if any)
If you feel like treating him then buy a carton of the kitten milk or some soya milk to mix in it.
They also like a Cheerio or a few rice crispies too
To make porridge you can buy a box of readybrek and take two teaspoons and mix to a paste with boiling water then cool and serve.