C++ Help (Hooking a function)

Say i have the sdk of a application with such a function defined.

void Check_Registration (
int Registered,
int Licenses,
int LicenseType,
int ChkSum,
int RESERVEDA,
int ( *Callback_CheckDate )( DATE *pDATE ),
void ( *Callback_NotifyOnline)( DATE *pDate, int ChkSum ) )
{

}

Now, i want to hook the NotifyOnline with my own function so i define.

typedef void (*NotifyOnlineOrig)(DATE *,int);

and my function:

void NotifyOnlineNew ( DATE *pDate, int ChkSum)
{
     NotifyOnlineOrig(pDate,ChkSum);
}

and try this inside the function.

void Check_Registration (
int Registered,
int Licenses,
int LicenseType,
int ChkSum,
int RESERVEDA,
int ( *Callback_CheckDate )( DATE *pDATE ),
void ( *Callback_NotifyOnline)( DATE *pDate, int ChkSum ) )
{
   NotifyOnlineOrig=Callback_NotifyOnline;
   Callback_NotifyOnline=NotifyOnlineNew;
}

of course the = parts dont work, how can i achive this, (its not a real sdk, so plz dont ask)

Thanks