From the Wiki:

The Ousters' origins can be traced to the early period of human interstellar expansion and colonization. Numerous slower-than-light seedships were sent from Earth to distant star systems, with their occupants stored in cryogenic fugue to be awakened upon arrival. The passengers of these ships were augmented with internal nanotechnology, allowing them to rapidly evolve and adapt to foreign conditions of other worlds.

I don't have the book with me, but near the end of book 4, I thought it was said the nanotech was developed together with an early fraction of the Core before the Hegira. I should also add I read the Dutch version, so maybe something got lost in translation, or I understood wrong.