Use Scenario Editor -> Edit Scenario -> Add Part -> Forced Trait -> Pick any caster class you like;
Don't forget setting to Player Characters only, otherwise you'd get all pawns to have that class.

Alternatively,
- Create ideology + use ritual to grant "Magically gifted" trait.
- Set social role for some colonist (other than one you'd want to grant talent to) to Void Seeker;
- Use Void Seeker to perform ritual;
- Get inspiration;
- Perform ritual to accept that arcane path;
- Done;

This takes two colonists though, and its not from the start, but still legit way.