Do you know if your old server uses Maildir or mbox format as storage? If you are not sure, please post a "ls" from the directory were the mails are stored.
1) If its maildir format, then you can copy the maildir right into the maildir folder of the ispconfig email account.
2) If its mbox, then there are scripts in the internet to convert to maildir, e.g. the script mb2md
3) If your old server provides imap, then you can use the application imapsync to copy the emails. This is indepandant of the local mail storage format and should work with any imap capable server.