Let's say you no longer wanted to use gmail to manage your mail, and wanted all your mail to go to your server instead and deleted from gmail. Is there such an animal that I can configure on my ISPCONFIG that will pull email fvia POP3 from another accoutn and add it into tmy ISPCONFIG email account?
etchmail is the right choice if you want to fetch mails regularly. In case you want to migrate a whole imap mail folder structure just once, then you might want to take a look at tools like imapcopy or imapsync. Onetime imap migrations can even be done in your mail client (e.g. thunderbird), add the google account and your new account and then drag & drop the folders from one account to the other. Just note that the data will flow from google to your desktop and then to your mail server, so this might not be the ebst way if you have dozens of gigabytes of mail data
Yes, I have setup fetchmail and yes I want to do it regularly. I expect to use thunderbird as my repository and delete all messages from gmail.