If you already created an account with an email login and want to use Facebook to login, as long as you are using the same email for your account and Facebook, you should be able to use both logins when you attempt to login with Facebook.
If you already created an account with an email login and want to use Facebook to login, as long as you are using the same email for your account and Facebook, you should be able to use both logins when you attempt to login with Facebook.