I don't believe a direct conversion is possible, however see http://forums.autodesk.com/t5/AutoCAD-Civil-3D/Riv
Does anybody know if it is possible to convert an autocad surface to civil 3d surface?
The easiest way is to explode the Acad surface, getting a bunch of 3Dfaces.
Finally you add those 3Dfaces to a new C3D surface definition.
When you add 3D Faces, make sure to check off "Maintain edges"
Unless you really don't want to.
Credit where credit is due! Give kudos or accept as solution whenever you can.