That depends on the shape of your surface, and if you are working with CATProduct or a CATPart file. A picture of your design would help.
In many cases, you can use the CloseSurface command to convert a Join (all the surfaces joined into a single surface with no openings) into a solid.
Often a single surface can be converted into a solid using the ThickSurface command.
There are other methods, but these are two of the most common ways to turn your surface geometry into a solid.