Hi,
Nope, sorry there isn't an easy way to do this. The xml used by the logbook looks very similar to GPX, so it is really unfortunate that Garmin simply did not chose to support GPX instead!
I'd email them... but there are other things to complain about their pitiful logbook program as well. I know that "GPSVisualizer" can read the XML files, though I don't know if there is anyway to get back a GPX or other format file.