You can do this on 10.8 server using dovecot commands. http://wiki2.dovecot.org/Tools/Doveadm/Mailbox
For example your "JunkMail" folder can be created for all users using.
sudo doveadm mailbox create -A JunkMail
If you setup roundcube on the 10.8 server it won't show up in the list of folders. Just go to the "manage folders" section and check the folder to show up in the list.