Try this
[DllImport("user32.dll")]
private static extern int GetSystemMenu(int HWND);
[DllImport("user32.dll")]
private static extern int AppendMenu(int MenuHandle, int Props, int
FlagsW, string text);
public const int BYPOSITION = 0x400;
//first get the handle of system menu
int hWnd = GetSystemMenu(myForm.Handle.ToInt32());
//append menu
AppendMenu(hWnd ,BYPOSITION,12345,"My new menu");