I think they prefer war orphans from the lower/less wealthy nobility as upward mobility is heavily frowned upon by the Hegemony. Still it might be possible so long as you at least adopt a free citizen. So no helots.
There was some interesting potential with the helot dad apparently having been in the same situation as Dann before, except with a “noble” slightly less sanguine then Calea is a while back. But Havenstone ultimately chose to keep the story about the unknown brother firmly in the past but it could potentially have been interesting to explore the helot mc having had a half-sibling raised as a “noble” as a partial subversion of the Karagond caste and chastity strictures.