I can't find any documentation on integration oauth2 services to ubuntu online accounts. Trying to add http://vk.com support. Chosen facebook plugin as example. Here is documentation https://vk.com/dev/auth_mobile
From the (semi-)official wiki (emphasis mine):
This time saving comes at a cost: the mental complexity of dealing with a separate thing, “Online Accounts”. Therefore, Online Accounts should be involved only where it is reasonably likely to save time. So it should not be used to handle special-purpose accounts that are only ever used for one app; that would make the overall experience more complex for no gain.
Note that the page mainly deals with the mobile implementation; but the general advice applies to the desktop one too. I would ask you to reconsider whether you really need to integrate with UOA.
If you do decide to go ahead with it, though; AFAIK, the user must first have installed an app that registers that is utilizes some service; then you can create a new account for that service in the Settings section.
Unfortunately, I don't think it allows 3-rd party apps to integrate arbitrary services yet (the circular data consumption would make for a 'kitchen sink', as it's called).