This feature is already there. You can export .gpx flight trails via History > Show Past Flight Trail... > [select flight trail] > Export
Depending on your device and how you use it, it may take a little bit to figure out how to get it off the device and into your favorite visualizer app, but once you figure it out the first time it should be pretty easy after that. On my Android devices I export to a Google Drive location that I can easily access from any of my other devices. Windows devices export directly to a specific iFly-managed folder on the C: drive (C:\_iFlyGPS_Data\User\#FlightTrails\#Export). I think the iFly devices export to the SD card, and then you have to transfer the SD card to another device and copy it off. I don't know how iOS devices work.
If for some reason you need it to be a .kml file, there are free .gpx-to-.kml converters on the web.