Yes! Simply login and scroll down to the “Alternate Email Addresses” section.
Also, if you upgraded one of your email addresses all of your other addresses are included in the upgrade.
When you link your accounts, make sure to login with the email you wish to be the primary email. If you have upgraded one of your email addresses to a premium subscription, make sure to login with that address.
Once you are logged in under the primary account, link your other emails. (You can always switch the primary email address later – which is an option on your general settings page.)
Once linked, all settings, account upgrades, pending followups and the calendar feed will share the same information as the primary account. The followups from the linked account are permanently moved to the new owner account. The followups themselves, however, will always be sent to the email from which they were scheduled.