There is a surface spline tool that will let you transform a polyline, etc., into a basic spline, but I think what you're really looking for is subdivision surfaces. Subdivision surfaces basically let you make smooth curves from flat surfaces. (A better explanation:
http://en.wikipedia.org/wiki/Catmull...vision_surface)
You can change the subdivision level of an object either from the Object menu, or in the Object Properties on the Tools menu.