If you plan to use CO points just remember that the Croatian Flag carrier "Croatia Airline" is partner of Star Alliance.
Croatia is a regional carrier and does not operate long haul flights. They have recently activated some codeshares with SAS for flights between NYC and Zagreb via Copenhagen.
Other solution is to take any flight from US to any european hub where Croatia Airlines operates (basically any major European city) then continue your journey to Croatia using Croatia airlines to Split or Dubrovnik (in the summer season they operates many flights).
It may be usefull to check the Staralliance booking tool on www.staralliance.com
If you want to use Sky team you have only one option with Alitalia from Rome to Dubrovnik.
Otherwise AirFrance has daily flights from Paris to Zagreb only.