External POP3 and IMAP consolidation (fetchmail)

Are you able to set a poll interval to automatically check for new mail on my external POP3 or IMAP account so that I don't have to press the "Get Mail" button within Webmail?

No – the only way to poll the external account is to use the “Get Mail” button. 

Please note that externally collected emails are not spam or virus checked.

Instead of using the external account feature, you may consider asking the ISP who hosts the external mailbox to automatically forward all new emails, or inform your contacts that you are no longer using the old address and close the old account.