Yes, this is documented on the Stack Overflow blog and in the Help Center.
You can add an unlimited number of logins of your choosing from the account page, by clicking "Edit Profile & Settings" at the top of your profile, then "My Logins" at the left, and then "add more logins..."
If this prompts you to log in as a different user, then you probably already used that "new" way to log in earlier. In that case see How can one link / merge / combine / associate two accounts / users? (Anonymous / unregistered / cookie, or OpenID / registered).
You can also remove credentials you no longer wish to be tied to your account from this screen.
- If you are signing in with Google and want to add another Google account, see here for instructions.