I'am not aware of any existing import tools to import a zimbra configuration in ispconfig, so I fear you would eithe have to write a script that imports the data by using e.g. the ispconfig remote api or you create the domains and mailboxes manually in ispconfig.
If zimbra stores the passwords in Linux standard crypt format or cleartext, then you can insert them in the ispconfig db.
The mail data in the zimbra accounts is most likely stored as maildir. If thats the case, then you can copy the completete maildir content of a zimbra account to the corresponding directory in ispconfig and then chown the files and folders to user and group vmail.
The maildir data in ispconfig is stored in /var/vmail/domain.tld/user/Maildir/