Tell them that next time to consider buying a newer sentra transmission to put in there, like one from a late B14 because it'd be cheaper than a rebuild and probably a more reliable tranny for the money. I mean, they've already spent what the car is worth putting two trannies in it. Still not a bad thing considering what a newer car would cost. Anyway, to answer your question, no additive or fluid is going to extend the life of your tranny, nobody can make those kinds of claims or guarantees (see class action lawsuits against makers of Prolong, Zymol, and most every other additive manufacturer). I'd just make sure to keep the fluid topped off and if you notice it's consuming fluid, have it checked out. Either you've just had a run of bad luck with auto trannies, your driving is absuive to the tranny, or your mechanic can't rebuild a tranny to save his life.
