As hard as our is to climb up I couldnt see how you could 'add' another rung/step without also raising whatever you hold onto to get up on the roof. IE: a rung/step at same elevation as roof could be an accident waiting to happen, especially getting back onto ladder from roof. So I went and found a picture of the AF ladder.
Hokey Smoke.. what were they thinking?
Not only top rung twice as far as ours- the rack looks taller and further back.
Adding a rung definitely is what Id be doing & easily doable on the AF ladder. Appears it would end up at same spacing adding just before rails turn. Be close to our height. The link bb_94401 provided is how ours is made. Contacting AF might yield a rung option or source if different.