Roundcube is a web application to access a mail server, just like Outlook or Thunderbird are desktop applications for accessing a mail server.
If you want to access the mail server directly with some mail program other than Roundcube, that has nothing to do with Roundcube.
It may not be possible; it depends on whether your system administrators allow direct access to the mail server from the outside.
Contact your system administrators, or fellow users of your mail system (some of whom may already have figured out how to do it, if it is possible).