I just posted the following on the Opera forum and want to check here, too, for help:

Reluctantly, I have decided to migrate from Opera Mail to The Bat! I have used Opera "views" as mailboxes and would like to keep the thousands of e-mail messages I have organized as they are, which means converting Opera views to The Bat! mailboxes. It looks like I can export Opera messages to .mbs files, which I can then import into The Bat! by importing "From UNIX mailboxes".

My question is whether I can automate this process, as I probably have hundreds of views and sub-views to convert to mailboxes, or must I manually export and import each view?

Opera does not use mailboxes, but it does have filters called labels and views which can serve the same function.