Unfortunetly, there is no real way (that I know of) to just curve some specific polys. I've been using XSI for a couple months now, and the only way I've found to make something curved is to create a sphere, shorten it to whatever size you want (half a sphere, a quater, whatever works best for what you're modeling), then add it to your model. Or, if you want it to be flat on the top, use a cylinder instead of a sphere. If you just have a rectagular prism, you can manually select every vertex you want to "curve" and reposition them until you have something like a curve, but that is very time consuming.

Of course, I could be wrong, and there could be an easier way to curve things, but there are so many features in XSI that there is no way to find out what they all do...

Anyway, I hope this helps, and good modeling