In the Star Wars sequels, we find that the First Order is filling their ranks with slave labor conditioned from early childhood for loyalty. Is this the only way to become a First Order Trooper, or is it possible for adults, or at least older children, to voluntarily choose to enlist despite not being raised in the First Order's childhood regime? Reason is not relevant - maybe someone really hates the New Republic, maybe they think fighting for the First Order would be a stable job, maybe they just like guns and glory and are willing to fight for anyone if given the opportunity, etc. Is such a wannabe SOL, or is there a process for them to prove themselves and put on the helmet?
I am relatively certain that you can enlist. You could enlist in the Empire, and the First Order has a lot of propoganda as well that they wouldn't use if they didn't want soldiers to enlist.
Also, I do not think that Captain Phasma and other high-ranking officers would be promoted to their position from slavery.