After a bit of googling myself, I don't think it's possible to directly convert them, as it's a proprietary format with a (no-doubt) complex internal structure combining video with various control features. Most probably only DeskScapes can handle them. I did find this page
however, that says that DeskScapes also stores a backup in .wmv format on Windows, which is convertible. So it looks like you'd need an active version of the program running on Windows to get the video you need.
I suppose there might also be other Windows programs that can directly convert the format or extract the video streams inside that can run under wine, but I didn't search for any myself.