Hi Morten,
You’re headed in the right direction with the sign up or log in the user with a social network but where you’re bound to run into trouble is, if the zendesk account that you want your user to authenticate with is different than the one they used to sign up for your app, what will happen is that once you authenticate with zendesk, your user’s current logged in state will be replaced with the zendesk login email and an additional account will be created on the backend if it didn’t already exist.
I’m looking for a similar way of “linking” a social/email account to an existing logged in state so that data can be fetched seamlessly. I’m currently building a google contact uploader and am having trouble with bringing in google contacts from any gmail account regardless of whether they match the current user’s original login or not.
I’ve raised this issue on the following thread that you can follow: Connecting to External Services More Seamlessly
Will keep you posted as I make any further developments. Feel free to PM me as well if you want to discuss.