There is nothing special regarding IMAP with ISPConfig as ISPConfig does not configure the imap server at all. The Imap server is courier in your case I guess and courier supports adding and deleting of folders if your imap client supports it.
I'am using pop3 and imap with ISPConfig every day on several servers.
If your imap server dont work, its most likely a configuration problem and not a ISPConfig problem.
Have a look at your mail logfile for errors.