No, there is not a drain plug on the torque converter.
There is some mixing of old fluid with new in the converter, which is why I used the extra fluid to do the flush. The capacity of mine is 12.25 quarts. The 18 that I used was overkill. The purist will say that I still have some old fluid in there....but I will say that it is very little. The 15 quarts that other's have used is plenty enough to do a good job of changing the fluid.