There's no such thing as a "Preferred" walk on. Mainly because the academies don't have athletic scholarships. They do however have walkons. At the end of Basic Training, the coaches will announce that anyone interested in showing what you got is welcome to try. You'll basically attend a practice where they'll have you perform. They'll either let you be on the team or not.
Remember; because they don't give scholarships for football, they aren't limited to how many players they recruit. But they are still limited by NCAA rules to how many are on the team. So instead of the normal 20+/- recruits like a normal school, they will recruit 50-60 at air force. (Not counting walk-ons). Before the season starts, they will lose about 10 of those. The remaining freshman will usually play JV. (They play against FCS and Jr. Colleges). A couple of freshman will be on the Varsity (FBS D1 team). Usually about another 10 will quit because of the academics, military studies, and football is too much for them. That gets them down to about 30-35. After the season, during spring practice, they will lose another 10 or so. By the end of the 2nd year, there will be about 20 left from that class. By the senior year, there will be about 15-18 seniors on the team. It's a tough life for a cadet to balance school and athletics.
On the good side, it's not like losing a scholarship in a civilian school. If you're not on the team any more, you're still in school. Some athletes get through basic training and decide already that they don't want to do the academy's academics and the sport and they don't ever play. So yes, they allow walk-ons. It's basically a weekend tryout. They know already what they're looking for as in positions, size, speed, etc... Just a matter if you're what they're looking for. best of luck.