It softens hardened up rubber seals as well as the rubber lip seals in the trans so they seal better again instead of only with enough throttle pressure and after they get warm enough to seal inside the drums.
This may very well be a good way to diagnose your problem.
If it starts engaging better after the additive, then you know it's the trans and not the converter.
I'll let you research how hard of a job it is and where to get a converter.
I can do a basic converter swap in about 1.5 hours with basic hand tools and ramps and about 40 or so minutes on a lift with air tools.
It takes some people two days.:D