Of course, you can add all the water in the universe and they'll still not be 100% water. The water-percent increment just gets smaller and smaller, the more water you add.
This "potato paradox" illustrates the same effect, but in the other direction, where a small relative decrease yields a large absolute decrease.
You won't have a potato anymore. But you won't have water anymore either.
You'll have a black hole.
https://en.wikipedia.org/wiki/No-hair_theorem
We don't know if black holes have hair, so the answer to your question is "We don't know".
https://en.wikipedia.org/wiki/Black_hole_information_paradox
Or another way - given you pour water in at any constant rate forever, given any p% you want to dilute to, there will be a point in time where that dilution level is exceeded, and will remain so for every point in time after that.
At this point, you have 1lb of solids and 199lbs of water, or 99.5%. Then you cough round off, and there you are, 100%!