The holes in the transmission case that accept the brass fittings for the cooler lines are tapped NPT, or pipe thread. This is not a straight cut thread like on a bolt, but is tapered. I have never seen any kind of thread insert or helicoil repair for pipe threads. The way I see it you have three options if the female threads in the case are damaged.
1- drill and tap the hole to the next size pipe thread being careful to keep swarf out of the transmission.
2- drill and tap the hole to the next size hydraulic fitting, this will require an o ring with the fitting to seal.
3- weld up the hole and re drill it to the correct size pipe thread.
My 2 cents.