….
There are three choices I can see that result from Dragonborn entering the setting. 1) you retcon that they were around the entire time, but SOMEHOW no player or DM knew it while they were adventuring and creating adventures in Greyhawk. Not a good solution.
2) you just ignore that they popped in and don't bother to explain it. Probably the best of the bad solutions.
3) You have a world altering event, like they were imprisoned magically as a race for 10 bazillion years and just got released, came from another world en masse, or some other way, which is the most setting destructive method.