Add a function to the list of functions to be registered
BOOL AddFn( CXllFn* pfn );
Pointer an initialised function definition.
Returns TRUE if the call succeeds, and FALSE if it fails for any reason.
Adds the defined function to the list of functions that will be registered when RegisterFunctions() is called during the OnXllOpenEx() event.
This function should only be used before RegisterFunctions() is called, preferably during the OnXllOpenEx() event handler.