The mistake you are making when doing this is you are not to actively play the sim, not even for a moment just to bring them into CAS. This deletes the part that gives them that role. Instead what you do is from Manage World, find the household the sim is living in, take them straight to CAS from there. Go into CAS, and edit their outfits, hairstyle, and anything else.
When it comes to assigning the roles, the game will usually pick from the homeless section, or creating a whole new sim for that role, and also picking sims with certain traits that did not have a job to fill a role. I was a little curious about this a while back. I went through a game save and evicted quite a few sims, and noticed that very soon after that they were doing different NPC roles such as the bartender, librarian, and so on, even though they already had a job (that your sim can also work at). I'm sure there is more to this than that, but this is what I have discovered personally.
