I went deep into my archives to get my Dogies schematic, and I found the sheet that shows how to install the add-a-ball kit. Both that sheet and the schematic make clear that the kit has just one additional relay. Interestingly, the existing Shoot Again relay is retained to continue to represent an extra ball earned on the features, while the new Extra Ball relay represents an extra ball earned by score or a sufficient number of Cactus Juice points. I didn't study the reset of those two relays in detail, but it looks like you stack the two appropriately. Either relay lights the Same Player Shoots Again light. So it's not "true AAB" but at least you can get both one EB from the feature and one on score. If you get across more than one score level on a single ball, or get one score level and a Cactus Juice level on the same ball, you lose out.
The installation of the kit may not require any soldering, per the flyer, but you have to move several wires around, add a switch onto the switch stack of a relay, and change connections on the tie points that can be seen clearly at the top of this photo:
I think the relay comes with a tall-stack armature in anticipation of the extra switch being added. Uninstalling the kit would also be a chore. I did not see evidence that the kit was designed to be generic enough to be the AAB kit for every Bally 4-player that claims to have an "add-a-ball model" on the flyer (Dogies through Bally Hoo, I think).
As I think about it, a true AAB conversion should make the feature-earned extra balls increment the same stepper unit as the score-earned extra balls. Also, the reset sequence needs to step the Added Balls stepper down to zero, just in case the machine was unplugged at an inopportune time. See other threads about AAB implementation for additional ideas and issues.