Currently when the Domino mail account is configured and used in particular Location document of Notes Client, then we cannot configure a third party POP3 account on the same location document. So if the user wants to see both mails (Domino and third party POP3) in same mail file, then it is not possible as of now.
Notes Client can be configured as POP3 client however separately (not together with Domino mail account). So there is a feature request to support third party POP3 account configuration in the same Location and mail file of Domino mail account. This will add convenience to user to check both location mails at a common place.