MBS files are mbox files with an "mbs" file extension. Thunderbird uses mbox files (with no extension) to store messages for each of your folders. Sylpheed uses them too and supports saving and importing them. So does Claws Mail. You can install import/export tools in Thunderbird to import mbox/mbs files and then you can export them to another format if you want.
Or, use IMAP in Thunderbird, import them and then drag them to the IMAP Inbox folder to upload them. Then they can be fetched with POP/Exchange again. You can do the same thing in Opera too. Just create an IMAP account to connect to the same account. Then, drag the messages from the POP account to the "Inbox" IMAP folder. That way, you don't need to export and import at all in Opera.
Mbox is the only format Opera supports. In the "mail/store" folder (see Help -> About Opera), there's an mbs file for each message. You could find a free mbs to eml converter and convert the whole folder of files. Then, you could import the eml files if Outlook supports that.