Thanks to Lawrence Lee, Robert Occhialini and Aaron Cope, there are instructions and a script to download and run that allows you to import xml formatted blog entries from Movable Type and Blogger into Radio. You can read all about it here.
I had to import the xml file three times before the imported entries worked correctly, and even then there were some problems, but I think they are resolvable. It’s very very important that your system data and time, and the time and date setting in Blgger (or BloggerPro) are exactly as described MMDDYYYYHHMMSS. In my case that meant changing my Mac to use leading zeros. When I tried to import the xml file without the leading zeros setting, the dates were bizarre, ranging from 1904, to 2052.
Then, after getting the imported entries dates to display properly, and after telling Radio to republish the entire site, some of the past entries didn’t show from the public Home page; users got an error.
I was able to get most of the past entries to display by hand editing and republishing them—I wanted to use Radio’s Categories anyway, so I didn’t mind. But some entries are still not displaying properly. By now, I know it’s too much to hope for support directly from Userland.