Should this process be included in the Network Download code, which I have in a separate .cpp file or can it go right into the main.cpp file?
This question doesn't make a whole lot of sense. If you are downloading a file from some web site based on the example code, then you are presumably creating a local file that the download is stored in. You know the file exists, because you created it. The code in the DownloadManager's saveToDisk() method returns true if the save was successful.

In any case, the action of downloading a file and then doing something with the contents of the file are two completely unrelated things. If you want to import the CSV file into your database as soon as it is saved, modify the DownloadManager class to add a signal that gets emitted in saveToDisk once the file has been written. Pass the name of the file as a parameter of the signal.

In the class that imports the CSV file, add a slot and connect it to this signal. When the slot gets called, you can open the CSV file and begin your conversion.