Not that I’m aware of. Writing one would be reasonably trivial. Create a database/spreadsheet with your extension information in it and export the data to the Bulk Handler. You can then run the bulk handler through the AMI interface.
Once again, there’s nothing that I’m aware of. You can get the information through the Asterisk CLI interface by querying the Database command, or you can use the Bulk Handler and export the Extensions data to a CSV file.
Even if there was, it wouldn’t be a Windows Application. It would be a browser application, so it wouldn’t meet your first requirement.
I don’t remember the example from a few days ago, but search “bulk handler” in the forums and there should be an example from last week where someone showed how to do a bulk add from the Asterisk CLI.