Yes it's possible, but only if you use the pre-fill registration mode for a provider. Within CB Connect under the providers Registration tab set "Mode" to "Pre-fill". This will cause it to fill the registration with as much information as it can (note at this time pre-fill will not populate avatar) for them to do a normal registration allowing them to select a CBSubs plan and it will link their social account to their CB account so it can be used for login. The alternative is to ensure CBSubs is configured to allow free registrations without subscriptions within CBSubs > Settings > Global.

I plan to only add Facebook, Twitter, LinkedIn and Google. Is the pre-fill available for most or all the social media accounts?

The pre-fill usage is available for all of the providers.

Also, for the accounts that already exist - How can the user sync up the social media account to the existing CB account?

They need to first login normally then use the Link button within your CB Login Module. Then they can login using their provider account from the on. Optionally you can synchronize their provider account information on linking by setting "Resynchronize" to "Yes" under the Link tab of the provider in CB Connect.

1) User logs in using existing credentials and want to sync to a social media account (by the way can they sync to more than one if they choose?)

It doesn't synchronize on login unless you enable that under the Login tab and that's only if they login via the provider account. It doesn't synchronize on Link unless you enable that. Yes, it can synchronize from multiple providers and whichever synchronizes last can override previous data.

2) User clicks on one of the social media login links to try to login - My guess is the system will match up email address in the CB account with the social media account to sync them.

No it will not do this, because that is a massive security vulnerability. You can specify whatever email you want in most of the provider accounts. Facebook even lets you change what email is sent to the application directly in the permission dialog. What this means is I could hijack your account if CB Connect allowed this and it never will, ever. To link to accounts you must login normally then click Link. It will not allow linking the accounts by any other means. This is the only way to ensure they own both accounts, sorry. I understand it's an inconvenient step for some people, but this is a non-negotiable security implementation.

3) Once they are synced to one or more social media accounts, what happens to the password they used before? Will they be able to login in using the traditional method if they are so inclined?

Password is completely untouched for synchronizations. So yes they'll be able to login normally as well if they like.