Most character concepts can fit pretty well among an awful lot of careers, so I don't see why this is necessary. One of my players has a would-be Jedi Knight, who was formerly an Imperial Officer. There's no real reason why he would need to take a career or specialization from Age of Rebellion to reflect that. He made a Guardian with the Peacekeeper spec (or was it Warleader?). That doesn't sound much like an Imp, but he took ranks in Leadership and a few other key skills, and it works out just fine. He also could have easily taken the Warrior career, if he imagined himself a ruthess Aggressor or leading a squadron of TIEs as a Starfighter Ace.
Choose your Career and Specs based on what you want your character to become. Toss in a few skill points to show who he used to be.