The DK starter zone essentially stipulates that all PC Death Knights must have been raised by Arthas just prior to the events of Wotlk. They could, however, easily justify allied race DKs by having their quest chain begin on present-day Acherus. From there, they could either be:
1. Teleported to the original starter zone as a 'vision of the past', to understand what it means to be Ebon Blade.
-OR-
2. Put through a much smaller quest chain to learn the basics, before being sent out into the world.
I mean, the devs don't even need to massage the lore here, since the Ebon Blade is constantly raising new Death Knights. They just need to not be lazy.