-Former ATP instructor in JAX200-2001.
By far the bulk of instructors were hired from the ranks of the career pilot program. However, at the time I was a student, the airlines were hiring like crazy, there was rapid instructor turnover, and the company would hire others from outside the career pilot program as needed. In fact, my instructor at ATP had jumped ship from Comair and with good recomendations, had been hired at ATP. Several of my students (MEI/II) who had come from Embry-Riddle were hired also. Now, however, there is very little movement into the regional airlines and ATP can fill its' instructor needs from within. It can't hurt to try though, If you go through the CFI program, ask your instructor about the chances of his putting in a word for you. If you're going to be stuck instructing for a while until hiring picks up, you cant beat ATP.
Also, as far as I know, ATP will NOT hire anyone who hasn't gone through at least some flight program with them. They like you to be familiar with their procedures, as well as getting a good look at your skills.