You could find a metric screw that fits into the button top, but I wouldn't use a Dremel to round off the button. It will be extremely difficult to hold the file stationary enough to only hit the areas you want to remove. The file will want to move in and out; taking off metal you don't want removed.
It's safer to file off the hex points....here's the one I did in a rough stage of rounding.
Keep working it into round and down to the diameter you want. Use your fingers to tell how close you are getting. You can feel the bumps when you turn it. Then, use the micrometer to check the results. Anything to within .05mm is plenty round.