You would run the risk of a ratings hit if you assigned that new FA to AAA. You also don't want him to sit unassigned. Take moe's advice and clear some dead weight off the 40 and put that guy in ML. You don't even have to release them if you need them to fill out a roster. Just designate for assignment with waivers. If they get claimed, you lose them but don't have to pay. If they don't get claimed, you have them for AAA depth without eating a spot on the 40.