Can you mix almond milk with cow milk?

You can, but there is little reason to. Kittens should never be given cow's milk, even when mixed with water. Cats cannot digest the lactose found in cow's milk very well, and some can develop diarrhoea after ingesting it. It is much safer to just give plain water, or even add a little extra water to the kitten's wet food if you are worried it is getting enough water.