Not quite sure what you're asking, but I'll try to help. Like quin said, you can set up Mail such that it'll handle email for multiple accounts. Personally I have two Gmail accounts & one MobileMe.
Use File > Add Account to add new *mailboxes*, and tweak the settings under Mail > Preferences.
And yes, Mail can handle multiple mailboxes w/the same provider. I have two Gmail accounts (one primary, and one for long-term storage). Mail handles both (or all three) seamlessly. You can view messages for each account respectively, or mail for all accounts by selecting the Inbox icon. I just name the respective Inbox icons something meaningful so I know what I'm lookin' at.
HTH.