You can. You want to be sure the water you add for topping of is reasonably sterile. But yes it is fine to use top of water for cooling your wort.
It will probably be more effective and quicker to set your brew kettle in an ice bath though first before you put it in the fermentor and top it of. The smaller volume will require less energy to be removed from it to get down closer to pitching temps.
I have a slate sink in my basement, and a wire cooling rack for cakes. I set that in my sink, I put my kettle on the rack, I fill with cold water up to the point just before the kettle wants to float off the cooling rack and then I add some blocks of ice to the sink.
In about 10-15 min.s it's down to 90*F, I can put it in my fermentor, add cold top off water, shake to aerate, and it has dropped the rest of the way to ideal pitching temps.