Hi,
I think adding more gears to your auto wouldn't fix it if the real problem is lack of power due to the gas conversion.
The next step up from your BA 4 speed auto is the BF 6-speed auto, however that conversion requires full re-wiring, new
PCM, new propeller shaft, and a whole lot of parts. In other words better to trade it for something, and get another one rather than even thinking about conversion.
The only conversion that is fairly simple these days is to swap in a XR6Turbo from BA into your BA. You would need to replace the gas system to one that suits turbo. You should be able to find an engine & gearbox package from an XR6T for approx $ 4500,
Again, if you sell your ute, and purchase a turbo ute, then fit a sequential gas system you are probably where you want to be? Cheers