Both humans and Viera can become archers. New recruits can randomly join you. So, if any one of those two races joins, you can change them to archers

Also, unlocking new jobs requires a certain amount of abilities from previous jobs. If you see a job you don't have yet on your enemy, you can check their status and view the job description (with select, I think? It's been a while) - it'll tell you what the requirements are