Take a look at the Function SetTimer.
For example if you just want to send packets to all connected players you could declare an ID for your new Timer in CoreServer.cpp and call SetTimer with your new ID in "InitInstance(HINSTANCE hInstance, int nCmdShow)".
Keep in mind that you have to kill that Timer when closing the Core Server (e.g. WM_Destroy ind WndProc).
In the function "OnTimer" you just simply add your new entry and call the corresponding code.
I hope i could help you with that.
I'm not really experienced with the Flyff Source and still learning