As far as I am aware this can’t be done natively up to Notes 6.
There are 3rd party products such as popweasel from mailgate
These allow a server to use PoP3 to collect mail from multiple accounts.
Many of these products are quite expensive. Many also integrate many other features and are almost a mail server in themselves.
I mention Popweasel as it is simple and relatively cheap.
I’ve never used it, other than the trial version, but it has a lot of good comments in many forums.
If you’ve a large number of users however whichever product you use you will have to ask whether the effort of maintaining the ISP mail account mappings to the Notes ones are more bother than finding a proxy server and setting up some security to allow the Notes server to become a full Internet mail server