Sure, it’s possible. You can have multiple email addresses and email boxes, and handle them differently; some can be checked for email each time you start mail, others can be enabled only when you need to. All your mail will stay in separate mailboxes, too.
Have your email account information handy before you start. You’ll need your smtp server URL and your login and password for that account. You might also want to have your web browser opened to the support and help page of your email provider.
When you’re in Mail, go up to Preferences under the “Mail” menu. Click on the second to the left hand button called “Accounts.” In that box, you’ll see a pane to the left that lists the email account you already have. At the bottom of that pane, there is a little “+” sign. Click that to add a new account.
Apple Mail will walk you through filling in the information for the new account. Mail will also check connections to mail servers, too, so if you made a mistake, you’ll have a chance to change it.
Check your instructions from your email provider: you might have to enter your full email address for your email account, as opposed to simply your login. If you run into problems, help for mail is under the “Help” menu.
After you’re all set up, check the “Mailbox Behaviors” and “Advanced” tabs. These control how your email is left on the server, what you want to do with junk mail, and more. (You can change these for your existing email account, too.)
If you want to delete the account later, beware: in some versions of Apple Mail, when you delete an email account, all your email will be deleted, too. You’ll be warned by Mail when you go to delete that account, but beware. If you want to delete an email account but keep the email, you can copy the email boxes from ~/Library/Mail/ under the name of the email account to the desktop to back them up. After you delete the account, you can import the email back into Mail with the “Import Mailboxes” command under the File menu.