I couldn't see why not - although in a closed room it does tend to get quite humid, muggy and rainforest-like

so you might want to leave a door open etc (which of course would reduce the effectiveness). I probably wouldn't use anything additional in the water though.
I forgot to say before that we did find it helped with Natalie's eczema, as with a heatpump running in our house, the air gets very dry, so it was good getting some moisture back in the air in her room.