C++ - Maintaining ABI - Interface parameter passing

Question

I'm developing a library in DLL. To maintain ABI I have taken an approach to use interface which is exported to my client and implement that interface in my library. For my library to work I need large number of input parameters from user, for that I have declared a data class and taking the reference as input parameter in one of the interface function.

For example:

class CInputParam
{   
public:      
    int a;      
    //......            
    int b;       
};    

class ISample   
{    
public:   
  virtual void SetInputParam(CInputParam& )=0;   
  virtual void DoWork()=0;   
};

 class CConcrete : public ISample    
{    
};    

I have two question. 1) If in future I need to add any new input param say 'int c' then I will add into CInputParam, but this will break the ABI as I need to recompile all my clients. What could be the best solution?

2) In case if I need to add to a new virtual function, can I derive a new interface ISample2 from ISample, add the new virtual function and change the concrete class to derive from ISample2 (earlier derived from ISample).

class ISample2 : public ISample            
{       
public:         

    virtual void NewFunction()=0;           
};     

class CConcrete : public ISample2      
{       
};