I am thinking of something like a config, xml, or JSON file. But I have not found any such solution. Perhaps FileHelpers doesn't have this functionality outside of the compiled assembly, but this is a long shot in the dark. The situation is that I have several files to read from and/or to write to, that quite often change structures slightly; and also the need to add new ones to interact with when new customers are acquired. I have used the FileHelpers library succesfully before, but for the sake of not having to recompile a rather large application every time a customer changes their file structure, or a new one starts doing business with us, I need to be able to add, remove, or alter the FileHelpers structure class of a CSV, Excel, or TXT file (depending on customer needs), rather dynamically. Any guidance will be appreciated.
You can generate C# classes and compile them at runtime based on structure defined in a configuration file. See this SO question for different ways of doing it including the use of Roslyn toolset.
Another, perhaps easier approach will be giving FileHelper's own Smart Format Detector a try.
While you can pursue this path and eventually succeed, it seems that requirements of your system (customers that change their CSV structure now and then, new customers with new file structures, etc.) calls for using a different library that doesn't need pre-defined classes to read CSV data and can access each row and column on demand. You can have metadata about format of each customer's file, and read/write data flexibly on the fly.