You can convert a replay game to add-a-ball, but since the rulesets are different, it's not going to be the same. Winning a replay is more valuable than winning a free ball, so generally the award on a replay game is more difficult to achieve. On the other hand, I used to play a "Top Card" at the local arcade which was converted to AAB. Once the special was lit, you could murder it! I guess it would be true on "Sky Jump" as well, because on a replay machine, once the special is lit, it remains on for the entire game. Add-a-ball games reset when the ball in play drains.

I have a Free Fall that goes up to ten balls total. I'm wondering how that would work with the ball count stepper in a Sky Jump. Plus how would you keep track of extra balls on the back glass?

I used to have the Bally Bazaar in both Replay and AAB versions. The back glasses were the same except for the blanked out replay window on the AAB version. If I recall correctly, the AAB version used the match numbers on the backglass for the ball count.

Usually, the credit counter is used to tally the extra balls. It is rigged to interrupt the normal ball count unit if it is in any position other than zero. It can be done.
I've seen several replay games converted this way.