It will add to the humidity for sure. But I am not sure about their stability.
Adding rocks will only add to the problem of centre of gravity ( I am sorry viper
).Beside adding rocks is to increase the surface area presented for evaporation ( besides contact) and thus have to be above the level of water.
Lucinda Brown's suggestion is the best. We can buy one here for less that $50.
I used to have a few 8-10" trays filled with water ( and rocks) strategically placed, along with a humidifier and a fan.