If they're very thin hairline-type cracks that don't appear to be get worse over time, then they are from the concrete shrinking unevenly as it dried; and yes, you can put a layer of cement on top. You can add small attractive stones to the concrete mix, and then expose them by carefully hosing off the top layer of cement as it's setting; or you can brush a pattern patterns into the wet new concrete with a broom. Ask at the place where you buy the concrete mix and they'll probably be happy to tell you all about these and all sorts of other decorative concrete finishes.But if the cracks have recently appeared, are getting worse, or are big enough to get the edge of a fingernail into, then they are caused by some structural flaw in the concrete and will only get worse over time. You could cover them over with a thin layer of new concrete - but your new concrete will just crack too, in the same place, after a few months.If structural cracks are small or slow-moving, a patch may work. If they're big or growing fast, you'll need to fix the underlying problem first.