Do you want to access your AOL email account with the Mail.app or forward your AOL email to your .Mac account?
Via your .Mac account preferences, you can check another email account that is a POP type account which will deposit received messages for the POP type account in your .Mac account's Inbox mailbox but this cannot be done with an IMAP type account.
And I don't believe you can forward messages from an AOL email account to another email account via AOL account prefereces.
If you want to access your AOL email account with the Mail.app, check this link.
http://members.aol.com/adamkb/aol/mailfaq/imap/