How to get a scientist with Psionics expertise?

I would like to be able to research Psionic Theory, but as a Materialist empire I require a scientist with Psionics expertise to be able to research it. Is there a way for me to get such a scientist or increase my odds of getting one?

There is a guaranteed method to get Psionics as any ethos.

1. Survey Ocean worlds (you're looking for the anomaly Drops in the Ocean). Your empire's Native Interference policy must be Active or Unrestricted, it will never spawn if your policy is Passive.
2. Survey Black Holes (you're looking for the anomaly Rainbow in the Dark).
3. Do the anomaly Rainbow in the Dark with any Scientist (though ideally, one that doesn't have Paranoid or Maniacal). You can get the Paranoid trait from this anomaly.
4. Do the Drops in the Ocean anomaly from earlier, with the Paranoid scientist you just got.
5. Choose the Paranoid dialog options, once your Scientist gains the Maniacal trait you'll know you succeeded.
6. Wait about a week in game. You will get an alert that your Scientist is now a Psionics Expert.

Cheat method:

1. add_anomaly HOLE_RADIATION_CAT
2. add_anomaly WAT_PLANET_CAT
3. Do the two anomalies you just added to planets in the order you added them.

After checking the code it seems like it's impossible for materialists to research psionics: In order to research Psionic Theory you need to have a scientist with the trait "Psionic Expertise". In order to unlock the trait for the pool, however, you need to already have researched "Psionic Theory".

leader_trait_expertise_psionics = {
  cost = 1
  modification = no
  icon = "gfx/interface/icons/traits/leader_traits/leader_trait_expertise_psionics.dds"
  modifier = {
      category_psionics_research_speed_mult = @categorybonus
  }
  leader_trait = yes
  leader_class = { scientist }
  prerequisites = { tech_psionic_theory }
  opposites = { 
      "leader_trait_expertise_materials" 
      "leader_trait_expertise_rocketry" 
      "leader_trait_expertise_voidcraft"
      "leader_trait_expertise_industry" 
      "leader_trait_expertise_field_manipulation"
      "leader_trait_expertise_particles"
      "leader_trait_expertise_computing" 
      "leader_trait_expertise_new_worlds"
      "leader_trait_expertise_statecraft" 
      "leader_trait_expertise_biology"
      "leader_trait_expertise_military_theory" 
  }
  # higher chance to generate techs in category psionics (done in tech-script)

  ai_categories = {
      society
  }

  ai_weight = {
      weight = 100
  }
}

As far as I've been able to tell, Scientist traits are generated randomly, not weighted based on ethos -- so the chance of a spiritualist empire generating a Psi-Specialized scientist is exactly the same as a pacifist empire.

That said, there are two ways -- either you can get extremely lucky and hire someone with the Psi. specialty directly from the leader pool, or you can get extremely lucky when a scientist levels up, and have them randomly acquire the psionic specialist trait.