Hello and welcome to JustAnswer!
You don't need to have the trans rebuilt you need to have the leak fixed. The source of the leak needs to be found and repaired. Common leaks are the cooler lines, solenoid pack, pan, or axle seals. Once the leak source is found then it can be repaired. The solenoid pack would be the most expensive to replace out of all the likely causes, with the part being $200 and the labor time about one hour, the other repairs will be cheaper.
No, an additive isn't going to help prolong the life of the trans. A fluid additive that is meant to stop leaks will work on a seal leak - either an axle seal or front pump seal, possibly a leaking cooler line to an extent. The additive works by softening the rubber which may help some, but isn't a permanent fix. Also when a leak is as large as you are describing it's too large for an additive to help, plus it will all leak out quickly before it had time to work.
Additives and fluids other than ATF+4 aren't good for your trans either. The TCM relies on the frictional properties of ATF+4, and a fluid additive will change that. Putting an additive in can cause clutch chatter and wear and eventual damage.