There is very little that cannot be achieved if you are willing to spend a little time putting your own modules together.
One of the weaknesses of the currently authentication provider system is that it lacks any real mechanisms for providing the link back elements required to associate an existing user with other providers.
The other weakness of the current provider model is that it is ironically somewhat restrictive in terms of layout and design.
Most people these days one a single login page with all the oAuth links for all the providers on the same page - they dont want different tabs or odd workflows they want simple actions.
We have built a couple of different workflows for clients - that do what you are asking for - but it does require bypassing the existing provider system - which is fundamentally broken for this type of action.
Westa