Now, let's say I want to edit the default output of the cmdlet. Is that even advisable?
Not sure what you mean. You can use Add-Member to add additional fields to the objects emitted by Get-NaSystemLog. Or you can change the values directly (they're just properties), but I'm not sure why you'd need to do that. You can provide a custom format for the data. Essentially, you can't change how the cmdlet works, but you can manipulate and display its output however you like.