Again, this is a PowerShell, not a WinSCP question.
I'm not a PowerShell expert, but what you pass to the add_* method is a script block. The script block can contain anything, function call, method call, or even a complete implementation.

After reading the description of $session.add_FileTransferProgress, I had a question:

Does an event handler have to be a standalone function, or can it be a method inside a class?