Outlook Express allows for several email addresses per person. For each person in your address book, you can right-click on them, select 'Properties' then 'Name' and add further email addresses - one of them is designated as the default.
If you want the program user to have alternate emails the procedure is as follows:
From the menu toolbar, click 'Tools - Accounts - Add - Mail'. You can enter the details of the alternate address via the connection wizard that then follows.
You can create separate folders in your inbox for each address if you wish, otherwise all mail will be routed to the main inbox. If you use separate folders, go to 'Tools - Message Rules - Mail' and select 'New Rule'. You can then use several options to direct incoming mail. Eg "When the to line contains 'Fred' move to the 'Fred' folder".
I use this for several accounts on the same ISP. If you specify several addresses on different ISPs, it will (I think!) allow you the option to connect to each in turn.