Run a Mail-Box & POP3 account

We have an Exchange server 2000, I have some new users from another company on my network and domain. We want them to have an Exchange account and also us ther own company pop3 account as well. That way they can be part of our Exchange group. When they send out external e-mails we want them to show their pop3 account info Example

Our company: companyA.com

their company: another-company.com their e-mail should show up as username@another-company.com when e-mailing from our LAN. I know how to add both accounts but I cannot get it to do what I want. They are using Outlook2003. I have don this before in Outlook2000 but I don’t remeber how I did it. HELP????