Is it a commmercial incubator? There should be a way to add water without opening.
If it's a homemade bator and you have a wire floor that is up off the bottom, you can put a small hole in the side of the bator at the bottom and squirt warm water in the bottom. That's how I'm keeping my humidity up in my homemade bator. The water dish/sponge doesn't have enough surface area to keep the humidity up where it needs to be, but if I fill that 3 times a day and put a couple of ounces in the very bottom of the bator it stays up great.