Bob is absolutely right. Cement products cure via a chemical reaction with water. Anything you mix in that is not specifically approved for mixing with uncured grout should not be added! Especially non-water based things, such as most sealers. There are some types of latex admixtures that work fine to mix in with grout, as latex is water based. Those do have certain "sealing" properties for the final cured grout, but I have heard that they give inconsistent results, and I have never used them.
Just about anything that typically requires multiple steps is like that for a reason, and any "single-step" miracle compound is almost universally inferior (e.g. Shampoo + conditioner in one, or stain and varnish combined...etc.). Otherwise, nobody would buy multiple step compounds in the first place.
Grout, cure, then seal. Or use an epoxy grout, such as Spectralock, which is basically a plastic-like base that never needs sealer. But you pay a lot more for it.