No, unfortunately, you cannot combine or switch two Tin Can accounts. That's why it's important that you keep everything under one account and foster that account.
The Facebook/Google connection is a separate sign-up method from the email sign up process. Because the sign-up processes are two different methods, you are not able to switch between the two methods. You cannot add an email address to your existing account as well. The only solution is to create a new account with the email sign-up process. Please be aware that the newly created account will not include any of your previous user data from your old account.