If you're having trouble creating smaller mailboxes within Eudora proper, select multiple messages in the browser and save them together as text. Then run the MailConverter on the text file, and run the AppleScript with the newly converted .mbx file. Also, Eumorpha is supposed to provide similar functionality, but I could never get it to work for me. I have an enormous database of Navigator mail. Use two programs in Info-Mac, Mail Converter and Eumorpha, together, to achieve this.
Full instructions come with the programs.
You might have to increase the memory partition of the two programs to convert all your messages at once, though. Also support for importing Navigator 3 and 4 address books is in 2.0v3. Okay, so I wanted to try OE for a little while, just to see what it was like. However, I received quite a few emails while using it that I want to keep in Emailer. It would greatly simplify things to just send the whole database back to Emailer instead of just bits and pieces.