The last time I used MFC, I basically used std::string/wstring throughout and made use of the c_str() function and the CString constructor to pass the string when it was needed.

Otherwise, you can do it by writing the appropriate operator:
Code:
CString& operator<<(CString&, const CryptoPP::Integer&);
By returning the input CString as the function value, you will be able to chain them together. Of course, that means that you'll probably also want to implement other overloads, like operator<<(CString&, const char*) and so on.