I was also thinking about this situation.An easier way is to create a directory in your server where you published your flex application.Just upload that kml file to your server. This directory must be published on IIS.So that you can add that kml file as a kmllayer to your application like http://youserverip/thedirectory/xyz.kml. You can do this for kml files. But in flex you cannot access local directories because of security restrictions. You need to write a webservice or something like that to manage it. For kmz files,you can use also your webservice to extract the kml file from the kmz file to this directory.So that you can reach that kml file as a kmllayer.
...could this "converter" be available to customers to live behind their firewall?