Yes!  First log in to your account using your username and password.  You'll be prompted to convert your account.  Select one of the options -- Facebook, Twitter or Google to associate with your account.  Your old account is now converted to use the service you chose in order to log in.  Remember, when you log in from now on, use the service you selected.

The account conversion workflow will close on April 30, 2013. If you have not updated your account by April 30, you will be required to make a new account to interact with the community. Creating a new account will cause you to lose your comment history.