Google+ buttons don't currently show as an option next to hubs; it has been suggested in the forums a few times that HP consider this. There were Google+ buttons next to hubs in the past but they were removed due to lack of use.
However, since Google+ is becoming a more popualr social network, and with the rise of Author Rank, it would make sense for it to be reconsidered.
You *can* add a link to Google+ in your own profile though, go to the Edit Profile section and you can add your profile link there.
Hope this helps.