Yup. Easy. There are nuts welded on the inside of the frame so no drilling, etc. Were the bolts already in the frame, or did they come with the step rails? If not you should clean the threads of any road gunk that might have gotten in there.
P.S. If you ever offroad in your Xterra, it's best to take the rails off. Put the bolts back in the holes to keep mud/dirt out until you get back home.