Yes you can export complete mailbox from Roundcube just follow the steps:
1. Sign in to Roundcube webmail.
2. Expand all mailbox folders such as Inbox, Sent, etc
3. Go to Folder tab from the top menu.

4. Now select all folder which you want to export then click on Choose Action button.

5. Select Download [.zip format] from Choose Action drop down list.

6. A Pop-Up Window will appear on screen then Click OK.
Thanks