They used to (and still kind of do) hire people outside of their programs. Now, the problem is that they have guaranteed CFI jobs to a lot of the people in their programs and those people need to get 1500 hours. So instead of getting their commercial CFI at 250 hours, and then instructing for another 200-400 hours and getting hired by an airline, their instructors are needing 1250+ hours to meet ATP mins.
If you are a two year CFI and can teach CFI and have a decent amount of multi - espeically PA-44 time, then you'd have a fair shot. But I'd say the chances are low for getting hired as a new CFI.
About 2.5 years ago when I did my CFI there, I wanted to work for them in Altanta, and they didn't have any openings in Atlanta. And at about the same time, they lowered the monthly retainer from $1500 to $750, and supposedly upped the bonuses and hourly pay. I decided the pay was too crappy to go to one of their other locations.
Then I tried to do the part 61 thing in Atlanta, and the market for part 61 CFI's is terrible there. I was lucky to get 15 hours of flight time and maybe 20-25 billable hours per month. Then I got hired by a university and moved away from ATL (current job). At my current job, the pay is salaried (but not great by any means), and the flight time is only 350-400 hours per year, and there is almost no multi to be had, so I tried to get on at ATP about a year ago just to build time faster and get multi time, but they told me at that time they weren't hiring anyone outside of ATP that isn't a two year CFI. Now that I'm qualified to teach initial CFI's I might give going to ATP another try, because I've gotten a whopping 14.7 hours of multi in the past two years at my current job. ATP also pays the 2 year CFI's a decent salary and it would definitely be a raise. But they treat their CFI's like crap, and teaching their CFI school seems like it would be especially stressful.