hmmm.. lets see. Your question is blurry. But, I'll try to explain based on my understanding.
In VC++ SDI (Single Document Interface) projects, there are several predefined classes. This is the skeleton of the project, they are the Application Class (class name end up with App), View Class (class name end up with View), Document Class (class name end up with Doc), and the main window class (CMainFrame). These classes are used to represent the whole application that you're building. Data are stored in the Document Class and the "rendering/view" of the data is presented/manipulated by using the View Class, thus you can decouple the view of the data from it's "rendering/presentation".
As for adding new class, it's just as usual. Declare, implement it in separate file. Unless you are deriving from MFC classes, you won't have anything generated by VC++.
I think you should learn more about the Document/View architecture employed in MFC. Using VC++ with SDI means you are using MFC. So, you have to grasp the whole concept before messing up with it. And you should read MSDN for the class hierarchy and it's interfaces (exported functions, virtual functions and pure virtual functions/ADT)
goodluck.