There are many chicken diseases that cause sneezing and runny noses. It is probably not avian influenza, as this is very rare. More likely something like MG, MS or Coryza. Please separate out the new bird, and any sick birds immediately. Get them far away from the rest of your flock. Your best bet is treating the affected birds with Tylan. Having an autopsy done on any birds that die is a good idea--so you know exactly what you are dealing with. Keep the dead birds refrigerated until you can get them to the vet or extension agency.
Please let this be a lesson on the dangers of introducing grown birds, and not doing quarantine. Though quarantine will NOT guarantee that the new bird, who may appear completely healthy, won't introduce some disease that your birds have no immunity to. That is why I will no longer bring in grown chickens.