First take a look at ParserConfiguration and what it allows you to change. if this is not enough...
You don't like how FluentArgumentParser generates names of verbs and parameters from your types and properties?
Implement INamingStrategy and pass it to the corresponding overload of ParserFactory.Create.
You don't like how property/parameter types are formatted in the help or how examples are generated?
Implement ITypeFormatter or IExampleValueProvider and pass it to the corresponding overload of ParserFactory.Create.
This should hopefully never become necessary, but if you want to customize the parser itself, you can do that, too.
Implement ICommandLineParser (or a decorator for it) pass it to the corresponding overload of ParserFactory.Create.