You can't make them smoother with a sealer - even epoxy floor fillers for leveling can't take the abuse of daily wear like concrete. You can wax and buff but it won't make them smoother. There are special concrete waxes that are full of color but they must be applied within a short time of pouring so they are part of the matrix as it cures. You can restain them for a different color finish once the sealer wears off or is removed but you need to think of these like a honed finished stone. A polished finish would be dangerously slippery / even what you have is probably slippery when wet.

You can change the color, you can lay a floor over them, but you can't get them smoother and strong after they are laid and cured. If you tried to polish them you would expose more of the large aggregate in the mix and make them rougher. The fines in the mix rise to the top and cure in at finish - if you try to layer over, it will be too thin to be strong enough.