Shortcut, and a satisfying one: there is probably nothing to export at all. Roundcube is not a mail store, it is a web face over an IMAP server at your host. Every folder you built lives on that server. Outlook connects to the same server and inherits everything.

Add the account in Outlook using IMAP with your host's server settings, the same credentials Roundcube takes. Hosting providers publish the IMAP host and port on their support pages, commonly mail.yourdomain.com on port 993 with SSL. On first sync Outlook downloads the complete folder tree, every folder you organized in Roundcube appears exactly as built, because Outlook is reading the very same folders.

The export job only exists in one scenario: the account itself closing, hosting cancelled or domain moving, where the server copy will vanish. Then, after the IMAP sync completes, preserve everything locally by exporting from Outlook to PST via File, Open and Export, Export to a file, selecting the account's whole tree. The synced copy becomes your permanent archive independent of the host.

Either way, verify the sync depth before trusting it: some Outlook setups default the offline slider short, showing recent mail only with the rest a click away on the server. For a full local copy ahead of any export, set Mail to keep offline to All in the account settings and let the complete history download.