if it is outlook express you can create a new identity in following way
GO to file and click manage identity.and then create new identity button.
Then create a new identity and configure pop3 account in it.But here you use multiple account then you could take backup easily by every identity. because it will create a new file for every new identity
If it is MS OUTLOOK 2003 Then you will go to account settings in tools menu and view existing account detail and then click add button to add new pop3 account.But when your using multiple account in MS OUTLOOK then backup up the second account mails is very difficult. Becuase all account mails are in one file called outlook.pst.So its not restore when you are reconfiguring the mails after system reinstallation time