To make it fully automatic you would have to write a Mach3 Plugin DLL, at least to the best of my knowledge - not recommended unless you have experience with making DLLs or a strong desire to learn!
However, an easy solution could be to make the Excel VBA create the file in a predetermined place, then create a button on your screenset in Mach3 to load the file. e.g.
‘ Load the roadrunner demo file
LoadFile(“C:\Mach3\Gcode\roadrunner.tap”)
Or
‘ Load and run the roadrunner demo file
LoadRun(“C:\Mach3\Gcode\roadrunner.tap”)
Using a DLL you would export a function that could be called by the VBA. The function would use Mach3 plugin interface to automatically load the file and optionally start it automatically, if that's what you wish. However, as I said before, not for the faint-of-heart. I wouldn't recommend it unless you have a working knowledge of C++ or an iron will to learn from scratch (it won't be easy if you don't already know the language)! The plugin interface is not very well documented; you will find most information and examples pertain to motion control, as that is it's primary purpose.
I know this probably isn't what you wanted to hear, but hopefully this is of some help.
Chris