So then I try to add a trait but without prereqs and threshreq, the trait will disappear or be demoted.
have a look at the documentation Guide to adding new content to CDDA for first time contributors · CleverRaven/Cataclysm-DDA Wiki · GitHub
I was able to do it by making an effect-on-condition mod, simply have a trait that’ll basically function as a flag for the EOC, and have that trait as a part of the profession.