It is quite simple how adding new Special Abilities work: when you enter the Squad menu in the game, click on a player then on the “+” icon where it says “Special Ability”. You will be given a choice of various specializations, which depend on your player’s role. After you have selected one, the player will start “learning” the ability.
To master an ability and be able to use it, you’ll need to invest 40 or 50 skill points in the Special Ability alone, depending on the ability. Skill points are earned by organizing good training sessions and when playing a matches in different tournaments, including friendlies.