According to osso-email's sourcecode, it stores mail in ~/apps/email/Mail.

You can try the following from an xterm, while Claws isn't running:
cd ~
cp -r apps/email/Mail/inbox Mail/osso (replace the second Mail if necessary with the name of your mailbox if it's not "Mail" or "Mailbox").

Start Claws again, tap-and-hold on the mailbox root folder, and choose "Check for new folders". a folder "osso" should appear containing your Osso emails.

Tell me if this works, I'll make an importer.