If data integration between OPA and RightNow is required, then no, you can't use one OPA Hub for multiple RightNow sites. If there's no data integration, then you can embed an OPA interview (via iframes in a RN Answer) in as many different Customer Portals as you like.
In anticipation of your next question... I can't comment on whether it will be possible in the future. (Development team -- can you comment on this?)