I've done this a handful of times to no ill effect. I don't bother with distilled usually - spring water is just fine for adjusting volume at pitching time, and a lot less weird. In your case, though, since you're looking to dilute beer that's already taken off, it's probably a good idea not to introduce oxygen (dissolved in spring water for taste.) Make sure to sterilize everything: fermenter lid, bottle, and such. Avoid splashing in the new water, and definitely don't stir or shake the fermenter - anything you stir it with is a risk of infection, and agitating your beer will encourage oxygenation (tastes like cardboard). Worry not, it will mix.