[LaDidi]

If I understand, you want to hook the call of Check_Registration done by another ?
Check_Registration use a function pointer to a callback_function. So, inside its body, you have a call like : Callback_NotifyOnline ( , );

If this API is in a DLL (like its_SDK.dll) and if you want to hook this call :
1. you need to export the same API in your library (my_SDK.DLL) with the same protoype
2. call the old API inside your API (export OR LoadLibrary, GetProcAddress)
3. rename my_SDK to its_SDK and its_SDD to Original_SDK

If this API isn't in a DLL ie resolved by compiler, you have the source so you can do anything !